Ann C/in Biochem 1990; 27: 532-541
Review Article
Sex hormone binding globulin: origin, function and clinical significance Colin Selby From the Department of Clinical Chemistry, City Hospital, Hucknall Road, Nottingham NG5 lPB, UK
Sex hormone binding globulin (SHBG) is a glycoprotein possessing high affinity binding for 17 P-hydroxysteriod hormones such as testosterone and oestradiol. It is probably synthesized in the liver, plasma concentrations being regulated by, amongst other things, androgen/oestrogen balance, thyroid hormones, isulin and dietary factors. it is involved in transport of sex steroids in plasma and its concentration is a major factor regulating their distribution between the proteinbound and free states. Its detailed role in the delivery of hormones to target tissues is not yet clear. Plasma SHBG concentrations are affected by a number of different diseases, high values being found in hyperthyroidism, hypogonadism, androgen insensitivity and hepatic cirrhosis in men. Low concentrations are found in myxoedema, hyperprolactinaemia and syndromes of excessive androgen activity. Concentrations are also affected by drugs such as androgens, oestrogens, thyroid hormones and anti-convulsants. Measurement of SHBG is useful in the evaluation of mild disorders of androgen metabolism and enables identification of those women with hirsutism who are more likely to respond to oestrogen therapy. Testosterone: SHBG ratios correlate well with both measured and calculated values of free testosterone and help to discriminate subjects with excessive androgen activity from normal individuals.
SUMMARY.
Additional key phrases: dihydrotestosterone; testostyerone; oestradiol; sex steriod transport
Although the presence of a specific plasma androgen binding protein was postulated in 19581 it was not isolated until 1965.2 The same year an oestradiol binding protein was independently isolated' and by competitive steroid binding studies the two proteins were shown to be identical." It consequently became known as Testosterone-estradiol Binding Globulin (TeBG) but has since been shown to bind many other steroids and drugs. It is now more commonly known as Sex Hormone Binding Globulin (SHBG) but has also been called SBP (Sex Steroid Binding Protein) and SSBG (Sex Steroid Binding Globulin). The protein is present in the plasma of primates, rabbits, goats, cows, sheep, reptiles and amphibians.!
This paper was prepared at the invitation of the Clinical Laboratory Investigation Sub Committee of the Scientific Committee of the Association of Clinical Biochemists, but does not necessarily reftect their views.
ISOLATION, PURIFICATION AND CHARACTERIZATION SHBG is most readily isolated from late pregnancy plasma by affinity chromatography followed by further purification steps."" Monomeric SHBG is a glycoprotein containing 373 amino acid residues and three carbohydrate side chains." The total carbohydrate content is 2834%6,7.9 and consists largely of sialic acid (36%) and N-acetylglucosamine (33%). The molecular structure of circulating SHBG is in dispute, a variety of monomeric to four unit structures being suggested.Y'" The most obvious property of SHBG is its high affinity for steroids, SHBG binds most readily to planar steroids which have 18 or 19 carbon atoms and a 17Phydroxyl group." Binding occurs by Van der Waals forces and polar attraction and one mole of dime ric SHBG binds approximately one mole of steroid. 6,7,9 Steroid binding depends on both temperature and pH, being some 2·5 times higher
532
Downloaded from acb.sagepub.com at UNIV OF IDAHO LIBRARY on November 4, 2015
Sex hormone binding globulin at 4°C than at 37°C but is irreversibly destroyed above 60°C and below pH 5. The three most avidly bound naturally occurring steroids are dihydrotestosterone (DHT), testosterone (T) and oestradiol (E 2 ) . BIOSYNTHESIS AND HOMEOSTATIC CONTROL SHBG has been localized in the hepatocytes of adult monkeys" and is secreted along with cortisol binding globulin by cultures of the human hepatoma cell line HepG2. 13 Concentrations are higher in hepatic venous than arterial blood." Plasma concentrations are increased by oestrogen" and thyroid hormones" and reduced
100 1 P:Z:=:I:I:l .....1'TTTT1.,...,
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by androgens," possibly by regulation of hepatic synthesis. Recently, growth hormone, somatomedin-C, other growth factors and nutritional status have been suggested" as the primary homeostatic mechanism for control of SHBG concentrations. In vivo growth hormone" and prolactin" reduce SHBG concentration whilst in vitro, insulin decreases SHBG production inhibiting the stimulatory effect of thyroxine and oestradiol." Dietary lipids affect SHBG concentration, which falls significantly in men after 2 weeks on a high fat diet." SHBG concentrations are lower in obese subjects of both sexes23,24 and rise following weight reduction. Congenital absence of SHBG has been described in two sisters."
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FIGURE I. The distribution of plasma dihydrotestosterone, testosterone and oestradiol between the free. albumin. SHBG and CBG bound states in normal subjects in different physiological situations. Results are expressed as the percentage of the total steroid concentrations circulating in each fraction. Data from Ref 28. • Albumin bound; 0 SHBG bound; • CBG bound; • free.
Downloaded from acb.sagepub.com at UNIV OF IDAHO LIBRARY on November 4, 2015
534
Selby
In human beings the biological half life of SHBG calculated from the rate of decline post partum is about 7 days." Little is known of the biological degradation although evidence" from rats injected with bovine SHBG suggests the sialic acid moiety is important in preventing rapid hepatic disposal. ROLE OF SHBG AND ALBUMIN IN STEROID TRANSPORT AND DELIVERY Less than 2% of biologically active steroids are free in the circulation, the remainder being bound to transport proteins, mostly SHBG and albumin. The bound and free fractions appear to exist in a state of dynamic equilibrium, governed overall by the Law of Mass Action; thus the size ofthe free fraction of a given steroid is dependent on: (i) Its total plasma concentration (ii) The avidity ofbinding to transport proteins (iii) The concentration of each binding protein (iv) The concentration and avidity of competing steriods Computer simulation of the distribution of some steroids between the free and protein bound states is summarized in Fig. 1.28 In normal men about 44%, and in normal women over 80% of the available SHBG binding sites remain unoccupied. Over 99% of the available steroid binding sites on albumin always remain unoccupied. This large excess of steroid binding sites means that changes in total hormone concentration produce relatively small changes in the size of the free hormone fractions. However, changes in SHBG concentration produce dramatic changes in both free and albumin bound hormone. Thus, the large increase in SHBG in pregnancy results in a rise in total testosterone (despite a fall in albumin concentration) and a fall in the percentage of free testosterone. Conversely, a reduction in SHBG concentration results in an increase of both albumin-bound and of percentage free testosterone. It follows that both absolute and relative protein concentrations play an important role in controlling the distribution of steroids between the bound and free states. A major metabolic role for both albumin and SHBG is thought to be that of a buffer store modulating fluctuations in steroid concentration and providing a reservoir of bioavailable hormone. Hitherto it has been assumed that it is only the lipid-soluble readily-diffusible free steroid which
is metabolically active. However, it has been argued that the concentration of some free steroids (e.g. oestradiol) is too low to generate a significant biological response" and it is possible that protein-bound steroid may also be available ---either dissociating in the immediate vicinity of the target tissue surface or entering the cell intact. A number of models'"?' depicting hormone delivery by dissociation have been proposed but the factors governing the kinetics of dissociation and tissue uptake remain contentious.P:" However in all situations studied to date, spontaneous dissociation of the protein-hormone complex is sufficiently rapid to account for the observed tissue uptake." Evidence is also accumulating that the intact SHBG-steroid complex interacts directly with certain target tissues. Prostatic" and decidual endometrial cell membranes" possess SHBG binding sites having characteristics typical of other membrane receptors, and SHBG and CBG have been located within the cytosol of a number of human and animal cells." These proteins may have a role in the specific transport of steroid to intra-cellular locations." Other roles for SHBG have been proposed: the metabolic clearance rate of steroids with a high affinity for SHBG is reduced.":" and it seems likely that SHBG protects such steroids from hepatic disposal. SHBG may also modify the rate of conversion of androstenedione to testosterone as this is higher in individuals with a low SHBG. 4 1 Finally, it has been suggested that differences in association characteristics of testosterone and oestradiol for SHBG may mean that increases in SHBG produce a more oestrogenic environment, i.e. oestrogen amplification." SHBG CONCENTRATIONS IN HEALTHY SUBJECTS In cord blood SHBG concentrations are low (approximately 30-40 nmol DHT bound/L) mean concentrations being higher in boys than girls." In male (but not female) infants SHBG concentrations are reported" to increase sharply from birth to 3 months before declining to normal pre-pubertal concentrations. Peak prepubertal concentrations (80-l00nmol DHT bound/L) are reached in both sexes between the ages of 5-7 years thereafter declining with approaching puberty, possibly as a result of increasing secretions of adrenal and gonadal androgens." However this pre-pubertal decline in SHBG still occurs in individuals with complete androgen insensitivity" and the prepubertal
Downloaded from acb.sagepub.com at UNIV OF IDAHO LIBRARY on November 4, 2015
Sex hormone binding globulin increase in insulin may be the major controlling factor." In male subjects adult SHBG concentrations (approximately 20-45 nmol DHT bound/L) are reached by about 18 years of age, but in female subjects adult concentrations (approximately 35-90 nmol DHT bound/L) are not reached until about 20 years of age. In both sexes SHBG concentrations slowly increase throughout life until by the mid-eighties they are about twice as high as those at 20.4S In healthy rested men SHBG concentrations have been reported to show a significant circadian rhythm characterized by a peak in the early afternoon and a nadir about midnight," although such a rhythm is difficult to reconcile with the biological halflife. SHBG has been shown by some authors to vary throughout the menstrual cycle being some 15% higher during the luteal phase" but this is not a consistent finding." Plasma concentrations of SHBG rise rapidly from conception" until the 30th week of gestation when they are 6-10 fold higher than nonpregnancy concentrations. 53 SHBG AND DRUGS Many different drugs modify circulating concentrations of SHBG and in such cases care is required in the interpretation of measurements of both testosterone and oestradiol. SHBG synthesis is increased by natural" and synthetic oestrogens," thyroid hormones, 16 anticonvulsants" and rifampicin. 56 In general there is an increase in SHBG and total plasma steroid concentration but a fall in the free fraction. However synthetic oestrogens produce a fall in total endogenous steroid due to suppression of steroid biosynthesis and negligible binding to SHBG. 2S Oestrogens administered orally are a more powerful stimulant of SHBG synthesis than those given percutaneously due to their direct absorption into the portal system." Antioestrogens such as clomiphene" and tamoxifen" also induce SHBG synthesis presumably as a result of their inherent weak oestrogenic activity. Dexamethasone," progesterone" and exogenous gonadotrophins'? have also been reported to increase SHBG concentration. SHBG concentrations are reduced by drugs with androgenic or progestagenic activity including natural and synthetic androgens," synthetic gestagens 63•64 (but not progesterone) and some anabolic steroids." Lowering of SHBG is thought to be due to a reduction in synthesis rather than increased metabolic clearance. Some drugs such as danazol" and norgestrel" displace
535
bound testosterone from SHBG and the resulting increase in free testosterone lowers SHBG synthesis. Others, such as medroxyprogesterone acetate do not displace endogenous steroids" and there is a fall in both total and free testosterone concentration." This difference is important if these drugs are used in combination with oestrogens for treating acne, hirsutism or virilism. SHBG CONCENTRAnONS IN DISEASE STATES Changes in SHBG occur in a wide variety of pathological conditions. Elevated concentrations are found in hypo gonadal men; both testicular failure and defective gonadotrophin secretion are associated with low plasma concentrations of total and free testosterone/" SHBG concentrations fall with androgen replacement therapy. In syndromes of androgen insensitivity both total testosterone and SHBG may be elevated,60,70 the degree of elevation being dependent on the severity of the defect and the degree of conversion of androgen to oestrogen (oestradiol concentrations are usually high). SHBG concentrations are elevated in a proportion of male diabetics" but the cause and clinical significance remain to be clarified. In men" (but not women?") severe alcoholic liver disease is associated with increased SHBG (possibly due to increased oestrone formation) and free androgen concentrations are reduced, partly due to high SHBG concentrations and partly to impaired gonadal function. In women high concentrations of SHBG are found in primary biliary cirrhosis." High concentrations of SHBG are found in women with anorexia nervosa" despite low oestradiol concentrations and failure to ovulate. In men with anorexia nervosa elevated SHBG concentrations are much less common" and refeeding has no demonstrable effect. Hyperthyroid subjects, including those overtreated with thyroid hormones, also have elevated SHBG concentrations":" probably due to increased hepatic synthesis, whereas those with myxoedema tend to have reduced levels. In women with breast cancer SHBG concentrations have been reported as normal" or low" and a relationship between tumour oestrogen receptor status and SHBG concentration has been observed." Reduced concentrations of SHBG and elevated androgens are frequently found in women with hyperprolactinaemiar" Prolactin and growth hormone are structurally and
Downloaded from acb.sagepub.com at UNIV OF IDAHO LIBRARY on November 4, 2015
536
Selby
phylogenetically related and SHBG concentrations tend to be low in acromegaly and subjects undergoing treatment with growth hormone." Grossly obese subjects of both sexes have reduced SHBG and total testosterone concentrations although free testosterone tends to remain normal." Increased androgen production rates are found in obese women; increased metabolic clearance rates are found in obese men and women possibly due to excessive androgen metabolism in adipose tissue." Syndromes of androgen excess A number of different clinical conditions are associated with an effective excess of circulating androgens. They present with acne," hirsutism." virilism" and features of polycystic ovarian disease. Biochemically they are a heterogeneous group characterized by increased androgen secretion from the adrenal, ovary or both," increased peripheral conversion and increased metabolic clearance rate of androgens'" and possible variation in target organ sensitivity." Total testosterone is elevated in only about 50%86,87 of cases, androstenedione and dehydroepiandrosterone and its sulphate may also be elevated" but SHBG is usually low, often less than 30 nmol DH T bound/L. Although there have been no detailed studies it is generally assumed that women who complain of acne and hirsutism in whom SHBG is normal are more likely to be suffering from an abnormality of the skin, e.g. elevated 511. reductase activity, than circulating androgen concentration." Management of hirsutism and acne is usually directed at reducing free androgen concentration by suppressing androgen secretion and/or increasing SHBG synthesis." Most physicians favour use of oestrogen (adminstered with a suitable progestagen to induce withdrawal bleeding) to increase SHBG synthesis. The powerful progestagen cyproterone acetate has several clinically useful effects. It suppresses gonadotrophin secretion and androgen synthesis and inhibits the effects of androgen on the target organ." However, the use of either oestrogen or cyproterone acetate is inappropriate in those who wish to conceive. Reduced concentrations of SHBG are also found in other conditions of androgen excess including congenital adrenal hyperplasia and Cushings syndrome." In women with androgensecreting ovarian tumours SHBG concentrations are variously normal" or low-presumably depending on the degree of peripheral conversion of androgens to oestrogens.
INDICATIONS FOR MEASUREMENT OF SHBG The diagnosis of hirsutism or virilism is made essentially on clinical grounds. Biochemical assessment merely helps to establish the aetiology of the condition. In milder cases of androgen excess characterized by acne, hirsutism and menstrual disturbance, total and free testosterone concentrations are likely to be normal in over 50% of cases. In a recent survey in Nottingham (Jeffcoate and Selby, unpublished observation) of 66 untreated women presenting with hirsutism, SHBG was below 30 nmol/L in 31 (47%) and below 35 nmol/L in 38 (58%) whereas testosterone was elevated (greater than 2·8 nmolj L) in only 18 (27%). When SHBG is suppressed and total testosterone remains normal or slightly elevated the ratio of testosterone to SHBG increases. Estimation of this ratio (usually termed the Free Androgen Index; FAI) and calculated as: total testosterone SHBG x 100 is a more sensitive means of identifying subjects with abnormal androgen status84,93- 9S than by measurement of either total or measured free testosterone (Fig. 2). In non-obese, non-hirsute oligomenorrhoeic women an elevated free androgen index during the early follicular phase of a spontaneous menstrual cycle is reported to be a sensitive and specific indicator of polycystic overian disease." Knowledge of SHBG concentration also enables a calculation of free testosterone (the 'derived' free testosterone") another extremely sensitive index of abnormal androgen status. The finding of a low SHBG is significant and has several important implications. (i) Total testosterone and SHBG concentrations are partially interdependent. A suppressed SHBG in the presence of a high normal or slightly elevated total testosterone infers an increase in 'bioavailable' androgen with a consequent increase in peripheral androgen activity. (ii) Since the aim of oestrogen therapy is to elevate plasma SHBG concentrations, those subjects who have a suppressed SHBG may be the most likely to respond. (iii) If SHBG concentrations lie within the reference range in women with hirsutism it is
Downloaded from acb.sagepub.com at UNIV OF IDAHO LIBRARY on November 4, 2015
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Review Article
Sex hormone binding globulin: origin, function and clinical significance Colin Selby From the Department of Clinical Chemistry, City Hospital, Hucknall Road, Nottingham NG5 lPB, UK
Sex hormone binding globulin (SHBG) is a glycoprotein possessing high affinity binding for 17 P-hydroxysteriod hormones such as testosterone and oestradiol. It is probably synthesized in the liver, plasma concentrations being regulated by, amongst other things, androgen/oestrogen balance, thyroid hormones, isulin and dietary factors. it is involved in transport of sex steroids in plasma and its concentration is a major factor regulating their distribution between the proteinbound and free states. Its detailed role in the delivery of hormones to target tissues is not yet clear. Plasma SHBG concentrations are affected by a number of different diseases, high values being found in hyperthyroidism, hypogonadism, androgen insensitivity and hepatic cirrhosis in men. Low concentrations are found in myxoedema, hyperprolactinaemia and syndromes of excessive androgen activity. Concentrations are also affected by drugs such as androgens, oestrogens, thyroid hormones and anti-convulsants. Measurement of SHBG is useful in the evaluation of mild disorders of androgen metabolism and enables identification of those women with hirsutism who are more likely to respond to oestrogen therapy. Testosterone: SHBG ratios correlate well with both measured and calculated values of free testosterone and help to discriminate subjects with excessive androgen activity from normal individuals.
SUMMARY.
Additional key phrases: dihydrotestosterone; testostyerone; oestradiol; sex steriod transport
Although the presence of a specific plasma androgen binding protein was postulated in 19581 it was not isolated until 1965.2 The same year an oestradiol binding protein was independently isolated' and by competitive steroid binding studies the two proteins were shown to be identical." It consequently became known as Testosterone-estradiol Binding Globulin (TeBG) but has since been shown to bind many other steroids and drugs. It is now more commonly known as Sex Hormone Binding Globulin (SHBG) but has also been called SBP (Sex Steroid Binding Protein) and SSBG (Sex Steroid Binding Globulin). The protein is present in the plasma of primates, rabbits, goats, cows, sheep, reptiles and amphibians.!
This paper was prepared at the invitation of the Clinical Laboratory Investigation Sub Committee of the Scientific Committee of the Association of Clinical Biochemists, but does not necessarily reftect their views.
ISOLATION, PURIFICATION AND CHARACTERIZATION SHBG is most readily isolated from late pregnancy plasma by affinity chromatography followed by further purification steps."" Monomeric SHBG is a glycoprotein containing 373 amino acid residues and three carbohydrate side chains." The total carbohydrate content is 2834%6,7.9 and consists largely of sialic acid (36%) and N-acetylglucosamine (33%). The molecular structure of circulating SHBG is in dispute, a variety of monomeric to four unit structures being suggested.Y'" The most obvious property of SHBG is its high affinity for steroids, SHBG binds most readily to planar steroids which have 18 or 19 carbon atoms and a 17Phydroxyl group." Binding occurs by Van der Waals forces and polar attraction and one mole of dime ric SHBG binds approximately one mole of steroid. 6,7,9 Steroid binding depends on both temperature and pH, being some 2·5 times higher
532
Downloaded from acb.sagepub.com at UNIV OF IDAHO LIBRARY on November 4, 2015
Sex hormone binding globulin at 4°C than at 37°C but is irreversibly destroyed above 60°C and below pH 5. The three most avidly bound naturally occurring steroids are dihydrotestosterone (DHT), testosterone (T) and oestradiol (E 2 ) . BIOSYNTHESIS AND HOMEOSTATIC CONTROL SHBG has been localized in the hepatocytes of adult monkeys" and is secreted along with cortisol binding globulin by cultures of the human hepatoma cell line HepG2. 13 Concentrations are higher in hepatic venous than arterial blood." Plasma concentrations are increased by oestrogen" and thyroid hormones" and reduced
100 1 P:Z:=:I:I:l .....1'TTTT1.,...,
._.
'--1KIl
E2
DHT
533
by androgens," possibly by regulation of hepatic synthesis. Recently, growth hormone, somatomedin-C, other growth factors and nutritional status have been suggested" as the primary homeostatic mechanism for control of SHBG concentrations. In vivo growth hormone" and prolactin" reduce SHBG concentration whilst in vitro, insulin decreases SHBG production inhibiting the stimulatory effect of thyroxine and oestradiol." Dietary lipids affect SHBG concentration, which falls significantly in men after 2 weeks on a high fat diet." SHBG concentrations are lower in obese subjects of both sexes23,24 and rise following weight reduction. Congenital absence of SHBG has been described in two sisters."
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T Men
T
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Women (Foil iculcr )
T
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Third trimester pregnancy
FIGURE I. The distribution of plasma dihydrotestosterone, testosterone and oestradiol between the free. albumin. SHBG and CBG bound states in normal subjects in different physiological situations. Results are expressed as the percentage of the total steroid concentrations circulating in each fraction. Data from Ref 28. • Albumin bound; 0 SHBG bound; • CBG bound; • free.
Downloaded from acb.sagepub.com at UNIV OF IDAHO LIBRARY on November 4, 2015
534
Selby
In human beings the biological half life of SHBG calculated from the rate of decline post partum is about 7 days." Little is known of the biological degradation although evidence" from rats injected with bovine SHBG suggests the sialic acid moiety is important in preventing rapid hepatic disposal. ROLE OF SHBG AND ALBUMIN IN STEROID TRANSPORT AND DELIVERY Less than 2% of biologically active steroids are free in the circulation, the remainder being bound to transport proteins, mostly SHBG and albumin. The bound and free fractions appear to exist in a state of dynamic equilibrium, governed overall by the Law of Mass Action; thus the size ofthe free fraction of a given steroid is dependent on: (i) Its total plasma concentration (ii) The avidity ofbinding to transport proteins (iii) The concentration of each binding protein (iv) The concentration and avidity of competing steriods Computer simulation of the distribution of some steroids between the free and protein bound states is summarized in Fig. 1.28 In normal men about 44%, and in normal women over 80% of the available SHBG binding sites remain unoccupied. Over 99% of the available steroid binding sites on albumin always remain unoccupied. This large excess of steroid binding sites means that changes in total hormone concentration produce relatively small changes in the size of the free hormone fractions. However, changes in SHBG concentration produce dramatic changes in both free and albumin bound hormone. Thus, the large increase in SHBG in pregnancy results in a rise in total testosterone (despite a fall in albumin concentration) and a fall in the percentage of free testosterone. Conversely, a reduction in SHBG concentration results in an increase of both albumin-bound and of percentage free testosterone. It follows that both absolute and relative protein concentrations play an important role in controlling the distribution of steroids between the bound and free states. A major metabolic role for both albumin and SHBG is thought to be that of a buffer store modulating fluctuations in steroid concentration and providing a reservoir of bioavailable hormone. Hitherto it has been assumed that it is only the lipid-soluble readily-diffusible free steroid which
is metabolically active. However, it has been argued that the concentration of some free steroids (e.g. oestradiol) is too low to generate a significant biological response" and it is possible that protein-bound steroid may also be available ---either dissociating in the immediate vicinity of the target tissue surface or entering the cell intact. A number of models'"?' depicting hormone delivery by dissociation have been proposed but the factors governing the kinetics of dissociation and tissue uptake remain contentious.P:" However in all situations studied to date, spontaneous dissociation of the protein-hormone complex is sufficiently rapid to account for the observed tissue uptake." Evidence is also accumulating that the intact SHBG-steroid complex interacts directly with certain target tissues. Prostatic" and decidual endometrial cell membranes" possess SHBG binding sites having characteristics typical of other membrane receptors, and SHBG and CBG have been located within the cytosol of a number of human and animal cells." These proteins may have a role in the specific transport of steroid to intra-cellular locations." Other roles for SHBG have been proposed: the metabolic clearance rate of steroids with a high affinity for SHBG is reduced.":" and it seems likely that SHBG protects such steroids from hepatic disposal. SHBG may also modify the rate of conversion of androstenedione to testosterone as this is higher in individuals with a low SHBG. 4 1 Finally, it has been suggested that differences in association characteristics of testosterone and oestradiol for SHBG may mean that increases in SHBG produce a more oestrogenic environment, i.e. oestrogen amplification." SHBG CONCENTRATIONS IN HEALTHY SUBJECTS In cord blood SHBG concentrations are low (approximately 30-40 nmol DHT bound/L) mean concentrations being higher in boys than girls." In male (but not female) infants SHBG concentrations are reported" to increase sharply from birth to 3 months before declining to normal pre-pubertal concentrations. Peak prepubertal concentrations (80-l00nmol DHT bound/L) are reached in both sexes between the ages of 5-7 years thereafter declining with approaching puberty, possibly as a result of increasing secretions of adrenal and gonadal androgens." However this pre-pubertal decline in SHBG still occurs in individuals with complete androgen insensitivity" and the prepubertal
Downloaded from acb.sagepub.com at UNIV OF IDAHO LIBRARY on November 4, 2015
Sex hormone binding globulin increase in insulin may be the major controlling factor." In male subjects adult SHBG concentrations (approximately 20-45 nmol DHT bound/L) are reached by about 18 years of age, but in female subjects adult concentrations (approximately 35-90 nmol DHT bound/L) are not reached until about 20 years of age. In both sexes SHBG concentrations slowly increase throughout life until by the mid-eighties they are about twice as high as those at 20.4S In healthy rested men SHBG concentrations have been reported to show a significant circadian rhythm characterized by a peak in the early afternoon and a nadir about midnight," although such a rhythm is difficult to reconcile with the biological halflife. SHBG has been shown by some authors to vary throughout the menstrual cycle being some 15% higher during the luteal phase" but this is not a consistent finding." Plasma concentrations of SHBG rise rapidly from conception" until the 30th week of gestation when they are 6-10 fold higher than nonpregnancy concentrations. 53 SHBG AND DRUGS Many different drugs modify circulating concentrations of SHBG and in such cases care is required in the interpretation of measurements of both testosterone and oestradiol. SHBG synthesis is increased by natural" and synthetic oestrogens," thyroid hormones, 16 anticonvulsants" and rifampicin. 56 In general there is an increase in SHBG and total plasma steroid concentration but a fall in the free fraction. However synthetic oestrogens produce a fall in total endogenous steroid due to suppression of steroid biosynthesis and negligible binding to SHBG. 2S Oestrogens administered orally are a more powerful stimulant of SHBG synthesis than those given percutaneously due to their direct absorption into the portal system." Antioestrogens such as clomiphene" and tamoxifen" also induce SHBG synthesis presumably as a result of their inherent weak oestrogenic activity. Dexamethasone," progesterone" and exogenous gonadotrophins'? have also been reported to increase SHBG concentration. SHBG concentrations are reduced by drugs with androgenic or progestagenic activity including natural and synthetic androgens," synthetic gestagens 63•64 (but not progesterone) and some anabolic steroids." Lowering of SHBG is thought to be due to a reduction in synthesis rather than increased metabolic clearance. Some drugs such as danazol" and norgestrel" displace
535
bound testosterone from SHBG and the resulting increase in free testosterone lowers SHBG synthesis. Others, such as medroxyprogesterone acetate do not displace endogenous steroids" and there is a fall in both total and free testosterone concentration." This difference is important if these drugs are used in combination with oestrogens for treating acne, hirsutism or virilism. SHBG CONCENTRAnONS IN DISEASE STATES Changes in SHBG occur in a wide variety of pathological conditions. Elevated concentrations are found in hypo gonadal men; both testicular failure and defective gonadotrophin secretion are associated with low plasma concentrations of total and free testosterone/" SHBG concentrations fall with androgen replacement therapy. In syndromes of androgen insensitivity both total testosterone and SHBG may be elevated,60,70 the degree of elevation being dependent on the severity of the defect and the degree of conversion of androgen to oestrogen (oestradiol concentrations are usually high). SHBG concentrations are elevated in a proportion of male diabetics" but the cause and clinical significance remain to be clarified. In men" (but not women?") severe alcoholic liver disease is associated with increased SHBG (possibly due to increased oestrone formation) and free androgen concentrations are reduced, partly due to high SHBG concentrations and partly to impaired gonadal function. In women high concentrations of SHBG are found in primary biliary cirrhosis." High concentrations of SHBG are found in women with anorexia nervosa" despite low oestradiol concentrations and failure to ovulate. In men with anorexia nervosa elevated SHBG concentrations are much less common" and refeeding has no demonstrable effect. Hyperthyroid subjects, including those overtreated with thyroid hormones, also have elevated SHBG concentrations":" probably due to increased hepatic synthesis, whereas those with myxoedema tend to have reduced levels. In women with breast cancer SHBG concentrations have been reported as normal" or low" and a relationship between tumour oestrogen receptor status and SHBG concentration has been observed." Reduced concentrations of SHBG and elevated androgens are frequently found in women with hyperprolactinaemiar" Prolactin and growth hormone are structurally and
Downloaded from acb.sagepub.com at UNIV OF IDAHO LIBRARY on November 4, 2015
536
Selby
phylogenetically related and SHBG concentrations tend to be low in acromegaly and subjects undergoing treatment with growth hormone." Grossly obese subjects of both sexes have reduced SHBG and total testosterone concentrations although free testosterone tends to remain normal." Increased androgen production rates are found in obese women; increased metabolic clearance rates are found in obese men and women possibly due to excessive androgen metabolism in adipose tissue." Syndromes of androgen excess A number of different clinical conditions are associated with an effective excess of circulating androgens. They present with acne," hirsutism." virilism" and features of polycystic ovarian disease. Biochemically they are a heterogeneous group characterized by increased androgen secretion from the adrenal, ovary or both," increased peripheral conversion and increased metabolic clearance rate of androgens'" and possible variation in target organ sensitivity." Total testosterone is elevated in only about 50%86,87 of cases, androstenedione and dehydroepiandrosterone and its sulphate may also be elevated" but SHBG is usually low, often less than 30 nmol DH T bound/L. Although there have been no detailed studies it is generally assumed that women who complain of acne and hirsutism in whom SHBG is normal are more likely to be suffering from an abnormality of the skin, e.g. elevated 511. reductase activity, than circulating androgen concentration." Management of hirsutism and acne is usually directed at reducing free androgen concentration by suppressing androgen secretion and/or increasing SHBG synthesis." Most physicians favour use of oestrogen (adminstered with a suitable progestagen to induce withdrawal bleeding) to increase SHBG synthesis. The powerful progestagen cyproterone acetate has several clinically useful effects. It suppresses gonadotrophin secretion and androgen synthesis and inhibits the effects of androgen on the target organ." However, the use of either oestrogen or cyproterone acetate is inappropriate in those who wish to conceive. Reduced concentrations of SHBG are also found in other conditions of androgen excess including congenital adrenal hyperplasia and Cushings syndrome." In women with androgensecreting ovarian tumours SHBG concentrations are variously normal" or low-presumably depending on the degree of peripheral conversion of androgens to oestrogens.
INDICATIONS FOR MEASUREMENT OF SHBG The diagnosis of hirsutism or virilism is made essentially on clinical grounds. Biochemical assessment merely helps to establish the aetiology of the condition. In milder cases of androgen excess characterized by acne, hirsutism and menstrual disturbance, total and free testosterone concentrations are likely to be normal in over 50% of cases. In a recent survey in Nottingham (Jeffcoate and Selby, unpublished observation) of 66 untreated women presenting with hirsutism, SHBG was below 30 nmol/L in 31 (47%) and below 35 nmol/L in 38 (58%) whereas testosterone was elevated (greater than 2·8 nmolj L) in only 18 (27%). When SHBG is suppressed and total testosterone remains normal or slightly elevated the ratio of testosterone to SHBG increases. Estimation of this ratio (usually termed the Free Androgen Index; FAI) and calculated as: total testosterone SHBG x 100 is a more sensitive means of identifying subjects with abnormal androgen status84,93- 9S than by measurement of either total or measured free testosterone (Fig. 2). In non-obese, non-hirsute oligomenorrhoeic women an elevated free androgen index during the early follicular phase of a spontaneous menstrual cycle is reported to be a sensitive and specific indicator of polycystic overian disease." Knowledge of SHBG concentration also enables a calculation of free testosterone (the 'derived' free testosterone") another extremely sensitive index of abnormal androgen status. The finding of a low SHBG is significant and has several important implications. (i) Total testosterone and SHBG concentrations are partially interdependent. A suppressed SHBG in the presence of a high normal or slightly elevated total testosterone infers an increase in 'bioavailable' androgen with a consequent increase in peripheral androgen activity. (ii) Since the aim of oestrogen therapy is to elevate plasma SHBG concentrations, those subjects who have a suppressed SHBG may be the most likely to respond. (iii) If SHBG concentrations lie within the reference range in women with hirsutism it is
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