0013-7227/90/1261-0186$02.00/0 Endocrinology Copyright © 1990 by The Endocrine Society
Vol. 126, No. 1 Printed in U.S.A.
Effects of 5,5'-Diphenylhydantoin on Thyroxine and 3,5,3'-Triiodothyronine Concentrations in Several Tissues of the Rat J. P. SCHRODER-VAN DER ELST AND D. VAN DER HEIDE Department of Endocrinology and Metabolic Diseases, University Hospital, Leiden, The Netherlands
ABSTRACT. We studied the effect of 5,5'-diphenylhydantoin
T4 level. Total T 3 was reduced in all organs. In tissues in which
(phenytoin, DPH) on the metabolism of thyroid hormones, the intracellular concentration of T4, and the source and concentration of T3. Two groups of six male Wistar rats received a continuous infusion of 10 ml saline/rat- day. One group received DPH in their food (50 mg/kg BW) for 20 days. For both groups [125I]T4 and [131I]T3 were added to the infusion fluid for the last 10 and 7 days, respectively. At isotopic equilibrium the rats were bled and perfused. Compared to the controls, plasma T4 and T 3 in the DPH group were reduced (22% and 31%, respectively); TSH did not change. The rate of production of T4 and the plasma appearance rate for T3 were decreased. Thyroidal T3 production was markedly reduced. From the increased [125I]T3/[126I]T4 ratio for plasma, it follows that total body conversion was enhanced. The tissue T4 concentrations decreased in parallel with the plasma
local conversion does not occur, i.e. heart and muscle, the decrease reflected the decrease in plasma T3. In the liver both plasma-derived T3 and locally produced T3 were diminished. In cerebellum and brain the plasma-derived T3 pool was even smaller than was expected from the decrease in plasma T3. This was partly compensated by an increase in local conversion. Only for these two organs was the decrease in the tissue/plasma ratio for [131I]T3 significant. Our results suggest tissue hypothyroidism, caused by a decrease in the production of T4 and T3, which is partly compensated by increased conversion in several organs. The transport of T3 into cerebellum and brain is disturbed, which can be attributed to the mode of action of DPH. (Endocrinology 126: 186-191, 1990)
P
in DPH-treated rats. [125I]T4 and [131I]T3 were infused simultaneously until isotopic equilibrium was achieved. In this way it was possible to determine accurately the intracellular concentrations of T 4 and T 3 and the contribution of local T4 to T 3 conversion to total cellular T3. Moreover, T4 and T 3 production by the thyroid could be calculated. The data for DPH-treated rats and controls were compared.
HENYTOIN, or 5,5'-diphenylhydantoin (hydantoin, DPH), is frequently used for the treatment of convulsive disorders and, occasionally, cardiac arrhythmias. Patients on DPH therapy exhibit decreased serum T4, frequently, but not always, decreased serum T 3 in most cases, and normal serum TSH levels as well as a normal response to TRH. They appear clinically euthyroid. Several explanations have been presented. DPH may decrease the protein-bound iodine concentration through competitive inhibition, since DPH competes with T 4 and T 3 for binding sites on T4-binding globulin, causing a decrease in the total thyroid hormone plasma concentration (1, 2). It has also been suggested that DPH can act as a thyroid hormone agonist (3-5) and/or can affect the conversion of T4 to T3, as has been shown in the liver in vitro (6). The present study was designed to investigate the intracellular T 4 and T 3 concentrations in various tissues in relation to the decrease in plasma T 4 and/or T 3 levels
Materials and Methods Animals
Received June 26,1989. Address all correspondence and requests for reprints to: Dr. D. Van Der Heide, Department of Endocrinology and Metabolic Diseases, University Hospital, Building 1, C4-R, Rijnsburgerweg 10, 2333 AA Leiden, The Netherlands.
Two groups of six euthyroid male Wistar rats (WI/U) were used. At the start of the experiment the body weight of the group of rats to be treated with DPH was 286 ± 19 g; that of the control group was 284 ± 15 g. The animals were individually housed in metabolic cages at 23 C, with alternating 12-h light and dark cycles. The rats were fed a semisynthetic American Institute of Nutrition (AIN) diet (7). This powdered diet was mixed thoroughly with water to a homogeneous paste. For the DPH group, 0.5 mg DPH/g food was added. The continuous iv infusions were administered at a constant rate (10 ml/day) via a cannula inserted into the right jugular vein and extended to the right atrium (8). The rats were unrestrained and could eat and drink normally. At the end of the experiment the DPH-
186
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187
DPH AND THYROID HORMONE METABOLISM treated animals weighed 354 ± 32 g, and the control rats weighed 348 ± 19 g. Thus, they exhibited a normal increase in body weight (DHP-treated rats, 69 ± 16 g; control rats, 70 ± 19 g in 20 days). Food intake for both groups was equal (DPH, 32 ± 3 g/day; controls, 33 ± 2 g/day). Based on the daily intake this resulted in a daily DPH dose of 49.2 ± 2.7 mg/kg BW. The serum DPH concentration, measured by the fluorescence polarography (FPIA) method (Abbott, TDx, Amstelveen, The Netherlands), was 1.01 ± 0.25 Radioactive iodothyronine infusions [125I]T4 and [131I]T3 (SA, 2200 and 3500 fiCi/^ig, respectively) were prepared in our laboratory (9, 10). [125I]Na and [l31I]Na were obtained from New England Nuclear (Boston, MA) and Amersham (Arlington Heights, IL), respectively; L-T3 and 3,5L-diiodothyronine, the respective substrates for labeling, were purchased from Sigma (St. Louis, MO). The purity of the tracers was assessed by HPLC. All infusions consisted of a sterile 0.9% NaCl solution containing 0.4 mg/ml ticarcillin (Ticarpen, Beecham S.A, Heppignies, Belgium) and 0.3 U/ml heparin (Organon, The Netherlands), with or without the labeled iodothyronines. The stock infusion solutions were stored at 4 C in the dark. The infusion flasks were protected from light to minimize artifactitious deiodination of the tracers. On the last day of infusion the contamination of [125I]T4 with [125I]T3 ranged between 0.10.24%, an amount that does not affect the results (11). No other labeled iodothyronines could be detected in [131I]T3. Design of the study The surgical procedure (insertion of the cannulas) was conducted on day 0. On day 8 DPH was added to the food (DPH group). On day 18 [125I]T4 (20 ^Ci/rat-day) was added to the infusion fluid (both groups). On day 21 [131I]T3 (25 ^Ci/ratday) was added to the infusion fluid (both groups). On day 28 the animals were killed. During the continuous infusion period (days 18-28), 24-h urine and feces samples were collected. The 125 I and 131I contents were counted and expressed as a proportion of the daily infused radioactivity. The animals were presumed to be in isotopic equilibrium when the amount of radioactivity in urine and feces together equaled the daily dose. Analytical procedures At the end of the infusion period, blood samples were taken from the tail under light ether anesthesia and collected in heparinized tubes. Propylthiouracil (PTU) was added to prevent artifactitious deiodination of iodothyronines to a final concentration of 0.1 mM (12). To free the tissues of trapped blood, the rats were perfused with 40-50 ml of a 0.9% NaCl solution containing 3 U heparin/ml and 0.1 mM PTU. Outflow of the perfusate was obtained by puncturing the inferior vena cava. The various organs were excised and placed on ice. Either whole small organs or weighed samples of tissues were minced and homogenized with a Potter homogenizer (B. Braun, Melsungen, West Germany) at 0 C in a 0.9% NaCl solution containing 0.1 mM PTU. The pituitary glands of two rats were pooled.
To determine the iodothyronine concentration, a measured aliquot was taken of each tissue homogenate and the plasma; the 125I and 131I contents were counted. The samples were extracted with ethanol in 25% ammonia (197:3, vol/vol) and 0.1 mM PTU. The dried extracts were dissolved in 0.1 ml 0.2 M ammonia containing carrier T4, T3) and KI (1 mg/10 ml) and put through HPLC to separate the iodothyronines. Analyses were performed with HPLC using a reverse phase C8 10 X 0.4cm column (Chrompack Middleburg, The Netherlands); the mobile phase was 0.625 M ammonium acetate, pH 3.8, and methanol (58:42, vol/vol), the flow rate was 0.6 ml/min, and fractions were 12 drops. The fractions collected were counted in a 7-counter, and their radioactivity was expressed first as a percentage of the daily dose and after recalculation as picomoles of T3 or T4 per g wet weight tissue. After decay of the 131I initially present in the samples the concentrations of stable T3 and T4 in plasma were assessed by RIA, using 131I-labeled T 3 and T4, respectively, as tracers. There was no interference of DPH (up to a concentration of 100 /xg/ ml serum) with these assays. Plasma TSH was measured by the specific RIA developed for the rat by the NIDDK, NIH. RP-2 was used as a standard. Protein was determined according to the method of Lowry et al. (13), with BSA as standard. The DNA contents of the various tissue homogenates were measured according to the method of Karstens and Wollenberger (14). Calculations The levels of T 4 , T 3 , tissue T 3 locally derived from T 4 [Lc T 3 (T 4 )], and tissue T 3 derived from plasma [pT 3 (T 3 )] were calculated according to the method of Van Doom et al. (15). In short, tissue T4 (pmol/g wetwt) tissue [125I]T4(% dose/g) X plasma T4(pmol/ml, RIA). plasma [12SI]T4(% dose/ml) The concentration of [125I]T4 in the tissue was corrected for trapped plasma (15). Lc T 3 (T 4 ) was calculated as follows: tissue [13lI]T3(% dose/g) x plasma [126I]T3(% dose/ml) plasma [131I]T3(% dose/ml) = tissue [126I]T3 from plasma (% dose/g) Then, tissue Lc [125I]T3 (% dose/g) = total tissue [125I]T3 (% dose/g) - tissue [125I]T3 from plasma (% dose/g) Thus, tissue Lc T3(T4) (pmol/g) tissue Lc [125I]T3 (% dose/g) x plasma T4 (pmol/ml, RIA). plasma [125I]T4 (% dose/ml) whereby [125I]T3 was multiplied by 2 to correct for the loss of 125 I from the distal ring of T 4 . The concentration of T 3 derived from plasma in the various
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DPH AND THYROID HORMONE METABOLISM
188
TABLE 1. Effects of DPH on plasma T3, T4) and TSH levels, PCRs for T3 and T4, PR of T4, and PAR for T3 and T3 production by both thyroid (Th PR T3) and peripheral conversion from T4
tissues was obtained as follows: tissue T3 from plasma (pmol/g) _ tissue [131I]T3 (% dose/g) x plasma T3 (pmol/ml, RIA) ~ plasma [131I]T3 (% dose/ml)
The total T 3 level in a tissue is the sum of the values calculated for tissue Lc T3(T4) and tissue T3 from plasma [T3(T3)]. The infusion rates for [131I]T3 and [125I]T4 and their respective blood levels were used to calculate the plasma clearance rates (PCR) for T4 and T3. If the plasma concentration is expressed as a percentage of the infused dose (per 1 h/100 g BW), then PCR (ml/h-100gBW): =
Endo• 1990 Voll26«Nol
100
% dose (h/100 g BW-ml) Then, the rate of production (PR) of T4 or the plasma appearance rate (PAR) for T 3 equals the PCR for T4 or T3 multiplied by their respective plasma concentrations. The production of T 3 in the thyroid (Th PR T3) can be calculated as follows. Since the amount of circulating T3 derived from T4 [T3(T4)] is given by:
and
T3(T4) (pmol/ml) x 100% = %T3(T4) T3 (pmol/ml, RIA) Then, (100 - [%T3(T4)] x PAR T3 = Th PR T3 (pmol/h/100 g BW) Statistical analysis was performed with Student's t test.
Results Each rat received the continuous infusion until the radioactivity in urine and feces reached a constant level and the total daily excretion of 125I and 131I equalled the daily input for at least 2 days. In both groups this occurred after 10 days of the [125I]T4 infusion and 7 days of the [131I]T3 infusion. At this stage urinary 125I and 131I accounted for 60-69% and feces for 31-40% of the total radioactivity excreted daily. There was no difference between the two groups. The rats were then assumed to be in isotopic equilibrium as far as the major pools of T4, T3, and their metabolites were concerned. The plasma concentrations of T4 and T 3 in DPH-treated rats had decreased. The plasma TSH concentration was normal. The PCR for T4 and T 3 did not change significantly. The PR of T4 and the PAR for T 3 decreased. Thyroidal T 3 production was almost zero, and peripheral T 3 production increased from 63% to 98% of the total T 3 production in DPH rats (Table 1). In liver, muscle, and cerebellum the T4 concentration decreased in parallel with the decrease in the plasma T4
Controls
DPH
Plasma T4 (nmol/liter) Plasma T3 (nmol/liter) Plasma TSH (ng/ml)
58 ± 3 46 ± 9
Vol. 126, No. 1 Printed in U.S.A.
Effects of 5,5'-Diphenylhydantoin on Thyroxine and 3,5,3'-Triiodothyronine Concentrations in Several Tissues of the Rat J. P. SCHRODER-VAN DER ELST AND D. VAN DER HEIDE Department of Endocrinology and Metabolic Diseases, University Hospital, Leiden, The Netherlands
ABSTRACT. We studied the effect of 5,5'-diphenylhydantoin
T4 level. Total T 3 was reduced in all organs. In tissues in which
(phenytoin, DPH) on the metabolism of thyroid hormones, the intracellular concentration of T4, and the source and concentration of T3. Two groups of six male Wistar rats received a continuous infusion of 10 ml saline/rat- day. One group received DPH in their food (50 mg/kg BW) for 20 days. For both groups [125I]T4 and [131I]T3 were added to the infusion fluid for the last 10 and 7 days, respectively. At isotopic equilibrium the rats were bled and perfused. Compared to the controls, plasma T4 and T 3 in the DPH group were reduced (22% and 31%, respectively); TSH did not change. The rate of production of T4 and the plasma appearance rate for T3 were decreased. Thyroidal T3 production was markedly reduced. From the increased [125I]T3/[126I]T4 ratio for plasma, it follows that total body conversion was enhanced. The tissue T4 concentrations decreased in parallel with the plasma
local conversion does not occur, i.e. heart and muscle, the decrease reflected the decrease in plasma T3. In the liver both plasma-derived T3 and locally produced T3 were diminished. In cerebellum and brain the plasma-derived T3 pool was even smaller than was expected from the decrease in plasma T3. This was partly compensated by an increase in local conversion. Only for these two organs was the decrease in the tissue/plasma ratio for [131I]T3 significant. Our results suggest tissue hypothyroidism, caused by a decrease in the production of T4 and T3, which is partly compensated by increased conversion in several organs. The transport of T3 into cerebellum and brain is disturbed, which can be attributed to the mode of action of DPH. (Endocrinology 126: 186-191, 1990)
P
in DPH-treated rats. [125I]T4 and [131I]T3 were infused simultaneously until isotopic equilibrium was achieved. In this way it was possible to determine accurately the intracellular concentrations of T 4 and T 3 and the contribution of local T4 to T 3 conversion to total cellular T3. Moreover, T4 and T 3 production by the thyroid could be calculated. The data for DPH-treated rats and controls were compared.
HENYTOIN, or 5,5'-diphenylhydantoin (hydantoin, DPH), is frequently used for the treatment of convulsive disorders and, occasionally, cardiac arrhythmias. Patients on DPH therapy exhibit decreased serum T4, frequently, but not always, decreased serum T 3 in most cases, and normal serum TSH levels as well as a normal response to TRH. They appear clinically euthyroid. Several explanations have been presented. DPH may decrease the protein-bound iodine concentration through competitive inhibition, since DPH competes with T 4 and T 3 for binding sites on T4-binding globulin, causing a decrease in the total thyroid hormone plasma concentration (1, 2). It has also been suggested that DPH can act as a thyroid hormone agonist (3-5) and/or can affect the conversion of T4 to T3, as has been shown in the liver in vitro (6). The present study was designed to investigate the intracellular T 4 and T 3 concentrations in various tissues in relation to the decrease in plasma T 4 and/or T 3 levels
Materials and Methods Animals
Received June 26,1989. Address all correspondence and requests for reprints to: Dr. D. Van Der Heide, Department of Endocrinology and Metabolic Diseases, University Hospital, Building 1, C4-R, Rijnsburgerweg 10, 2333 AA Leiden, The Netherlands.
Two groups of six euthyroid male Wistar rats (WI/U) were used. At the start of the experiment the body weight of the group of rats to be treated with DPH was 286 ± 19 g; that of the control group was 284 ± 15 g. The animals were individually housed in metabolic cages at 23 C, with alternating 12-h light and dark cycles. The rats were fed a semisynthetic American Institute of Nutrition (AIN) diet (7). This powdered diet was mixed thoroughly with water to a homogeneous paste. For the DPH group, 0.5 mg DPH/g food was added. The continuous iv infusions were administered at a constant rate (10 ml/day) via a cannula inserted into the right jugular vein and extended to the right atrium (8). The rats were unrestrained and could eat and drink normally. At the end of the experiment the DPH-
186
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187
DPH AND THYROID HORMONE METABOLISM treated animals weighed 354 ± 32 g, and the control rats weighed 348 ± 19 g. Thus, they exhibited a normal increase in body weight (DHP-treated rats, 69 ± 16 g; control rats, 70 ± 19 g in 20 days). Food intake for both groups was equal (DPH, 32 ± 3 g/day; controls, 33 ± 2 g/day). Based on the daily intake this resulted in a daily DPH dose of 49.2 ± 2.7 mg/kg BW. The serum DPH concentration, measured by the fluorescence polarography (FPIA) method (Abbott, TDx, Amstelveen, The Netherlands), was 1.01 ± 0.25 Radioactive iodothyronine infusions [125I]T4 and [131I]T3 (SA, 2200 and 3500 fiCi/^ig, respectively) were prepared in our laboratory (9, 10). [125I]Na and [l31I]Na were obtained from New England Nuclear (Boston, MA) and Amersham (Arlington Heights, IL), respectively; L-T3 and 3,5L-diiodothyronine, the respective substrates for labeling, were purchased from Sigma (St. Louis, MO). The purity of the tracers was assessed by HPLC. All infusions consisted of a sterile 0.9% NaCl solution containing 0.4 mg/ml ticarcillin (Ticarpen, Beecham S.A, Heppignies, Belgium) and 0.3 U/ml heparin (Organon, The Netherlands), with or without the labeled iodothyronines. The stock infusion solutions were stored at 4 C in the dark. The infusion flasks were protected from light to minimize artifactitious deiodination of the tracers. On the last day of infusion the contamination of [125I]T4 with [125I]T3 ranged between 0.10.24%, an amount that does not affect the results (11). No other labeled iodothyronines could be detected in [131I]T3. Design of the study The surgical procedure (insertion of the cannulas) was conducted on day 0. On day 8 DPH was added to the food (DPH group). On day 18 [125I]T4 (20 ^Ci/rat-day) was added to the infusion fluid (both groups). On day 21 [131I]T3 (25 ^Ci/ratday) was added to the infusion fluid (both groups). On day 28 the animals were killed. During the continuous infusion period (days 18-28), 24-h urine and feces samples were collected. The 125 I and 131I contents were counted and expressed as a proportion of the daily infused radioactivity. The animals were presumed to be in isotopic equilibrium when the amount of radioactivity in urine and feces together equaled the daily dose. Analytical procedures At the end of the infusion period, blood samples were taken from the tail under light ether anesthesia and collected in heparinized tubes. Propylthiouracil (PTU) was added to prevent artifactitious deiodination of iodothyronines to a final concentration of 0.1 mM (12). To free the tissues of trapped blood, the rats were perfused with 40-50 ml of a 0.9% NaCl solution containing 3 U heparin/ml and 0.1 mM PTU. Outflow of the perfusate was obtained by puncturing the inferior vena cava. The various organs were excised and placed on ice. Either whole small organs or weighed samples of tissues were minced and homogenized with a Potter homogenizer (B. Braun, Melsungen, West Germany) at 0 C in a 0.9% NaCl solution containing 0.1 mM PTU. The pituitary glands of two rats were pooled.
To determine the iodothyronine concentration, a measured aliquot was taken of each tissue homogenate and the plasma; the 125I and 131I contents were counted. The samples were extracted with ethanol in 25% ammonia (197:3, vol/vol) and 0.1 mM PTU. The dried extracts were dissolved in 0.1 ml 0.2 M ammonia containing carrier T4, T3) and KI (1 mg/10 ml) and put through HPLC to separate the iodothyronines. Analyses were performed with HPLC using a reverse phase C8 10 X 0.4cm column (Chrompack Middleburg, The Netherlands); the mobile phase was 0.625 M ammonium acetate, pH 3.8, and methanol (58:42, vol/vol), the flow rate was 0.6 ml/min, and fractions were 12 drops. The fractions collected were counted in a 7-counter, and their radioactivity was expressed first as a percentage of the daily dose and after recalculation as picomoles of T3 or T4 per g wet weight tissue. After decay of the 131I initially present in the samples the concentrations of stable T3 and T4 in plasma were assessed by RIA, using 131I-labeled T 3 and T4, respectively, as tracers. There was no interference of DPH (up to a concentration of 100 /xg/ ml serum) with these assays. Plasma TSH was measured by the specific RIA developed for the rat by the NIDDK, NIH. RP-2 was used as a standard. Protein was determined according to the method of Lowry et al. (13), with BSA as standard. The DNA contents of the various tissue homogenates were measured according to the method of Karstens and Wollenberger (14). Calculations The levels of T 4 , T 3 , tissue T 3 locally derived from T 4 [Lc T 3 (T 4 )], and tissue T 3 derived from plasma [pT 3 (T 3 )] were calculated according to the method of Van Doom et al. (15). In short, tissue T4 (pmol/g wetwt) tissue [125I]T4(% dose/g) X plasma T4(pmol/ml, RIA). plasma [12SI]T4(% dose/ml) The concentration of [125I]T4 in the tissue was corrected for trapped plasma (15). Lc T 3 (T 4 ) was calculated as follows: tissue [13lI]T3(% dose/g) x plasma [126I]T3(% dose/ml) plasma [131I]T3(% dose/ml) = tissue [126I]T3 from plasma (% dose/g) Then, tissue Lc [125I]T3 (% dose/g) = total tissue [125I]T3 (% dose/g) - tissue [125I]T3 from plasma (% dose/g) Thus, tissue Lc T3(T4) (pmol/g) tissue Lc [125I]T3 (% dose/g) x plasma T4 (pmol/ml, RIA). plasma [125I]T4 (% dose/ml) whereby [125I]T3 was multiplied by 2 to correct for the loss of 125 I from the distal ring of T 4 . The concentration of T 3 derived from plasma in the various
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DPH AND THYROID HORMONE METABOLISM
188
TABLE 1. Effects of DPH on plasma T3, T4) and TSH levels, PCRs for T3 and T4, PR of T4, and PAR for T3 and T3 production by both thyroid (Th PR T3) and peripheral conversion from T4
tissues was obtained as follows: tissue T3 from plasma (pmol/g) _ tissue [131I]T3 (% dose/g) x plasma T3 (pmol/ml, RIA) ~ plasma [131I]T3 (% dose/ml)
The total T 3 level in a tissue is the sum of the values calculated for tissue Lc T3(T4) and tissue T3 from plasma [T3(T3)]. The infusion rates for [131I]T3 and [125I]T4 and their respective blood levels were used to calculate the plasma clearance rates (PCR) for T4 and T3. If the plasma concentration is expressed as a percentage of the infused dose (per 1 h/100 g BW), then PCR (ml/h-100gBW): =
Endo• 1990 Voll26«Nol
100
% dose (h/100 g BW-ml) Then, the rate of production (PR) of T4 or the plasma appearance rate (PAR) for T 3 equals the PCR for T4 or T3 multiplied by their respective plasma concentrations. The production of T 3 in the thyroid (Th PR T3) can be calculated as follows. Since the amount of circulating T3 derived from T4 [T3(T4)] is given by:
and
T3(T4) (pmol/ml) x 100% = %T3(T4) T3 (pmol/ml, RIA) Then, (100 - [%T3(T4)] x PAR T3 = Th PR T3 (pmol/h/100 g BW) Statistical analysis was performed with Student's t test.
Results Each rat received the continuous infusion until the radioactivity in urine and feces reached a constant level and the total daily excretion of 125I and 131I equalled the daily input for at least 2 days. In both groups this occurred after 10 days of the [125I]T4 infusion and 7 days of the [131I]T3 infusion. At this stage urinary 125I and 131I accounted for 60-69% and feces for 31-40% of the total radioactivity excreted daily. There was no difference between the two groups. The rats were then assumed to be in isotopic equilibrium as far as the major pools of T4, T3, and their metabolites were concerned. The plasma concentrations of T4 and T 3 in DPH-treated rats had decreased. The plasma TSH concentration was normal. The PCR for T4 and T 3 did not change significantly. The PR of T4 and the PAR for T 3 decreased. Thyroidal T 3 production was almost zero, and peripheral T 3 production increased from 63% to 98% of the total T 3 production in DPH rats (Table 1). In liver, muscle, and cerebellum the T4 concentration decreased in parallel with the decrease in the plasma T4
Controls
DPH
Plasma T4 (nmol/liter) Plasma T3 (nmol/liter) Plasma TSH (ng/ml)
58 ± 3 46 ± 9
