Circulating T3 Concentrations Research Articles

Thermoregulatory dysfunction in post‐weaning congenic corpulent rats

To assess the capacity for cold induced thermoregulation in congenic corpulent rats, groups of normally fed post‐weaning lean and obese LA/Ntul//‐cp rats 6‐9 weeks of age were subjected to a 24°C vs. 4°C environment for < 14 hours. Measures of resting VO2 (RMR) and the thermoregulatory responses to cold exposure, the thermic response to noradrenaline, thyroidal parameters, and measures of core and peripheral body temperatures following cold exposure were obtained. Body weights of obese were greater than in lean rats (p=<0.01), and 14 hours of 4°C exposure demonstrated phenotype effects on both core and rectal temperatures (lean > obese, rectal p=0.0012, core p= 0.0038). The RMR at thermal neutrality and the thermic response to 45 minutes of acute 4°C cold exposure was greater in lean than obese phenotype at all time points measured. The thermic responses to 100 vs 200 µg NE s.c. were greater in lean than in obese rats. Cold exposure induced a significant increase in circulating T3 and the T3:T4 ratios but T4 concentrations were unchanged by cold exposure challenge. Phenotype effects on outer ring T4‐5’ deiodinase activity/mg protein were present in Kidney and Liver (Lean > Obese) while Interscapular brown adipose tissue (IBAT) demonstrated both phenotype and cold induced effects (Obese > Lean). In contrast, T4‐5’deiodinase activity in gastrocnemius muscle tissue was similar in both phenotypes and lacked cold induced effects (p= n.s.). The results of this study indicate that an impaired capacity for thermoregulation occurs early in the lifespan of the corpulent rat, despite a robust increase in total IBAT 5’deiodinase activity and in circulating T3 concentrations following cold exposure in both lean and obese phenotypes. While the physiologic basis for the elevated T3 concentrations following cold exposure remains unclear and are likely secondary to the combined effects of optimizing peripheral T4 to T3 conversion rates in both phenotypes, and possible changes in kinetics of T3 receptor binding and clearance in IBAT and other tissues and which may contribute to a greater caloric efficiency and propensity for weight gain in the obese phenotype.

Sleep deprivation alters thyroid hormone economy in rats.

What is the central question of this study? The relationship between the thyroid system and sleep deprivation has seldom been assessed in the literature, and mounting evidence exists that sleep disturbances influence human lifestyles. The aim of this study was to investigate the hypothalamic-pituitary-thyroid axis and thyroid hormone metabolism in sleep-deprived and sleep-restricted rats. What is the main finding and its importance? Central hypothyroidism and high thyroxine (T4 ) to 3,5,3'-triiodothyronine (T3 ) activation in brown adipose tissue were observed following sleep deprivation. Sleep-restricted rats exhibited normal thyroid-stimulating hormone and T4 concentrations despite increased circulating T3 . Sleep recovery for 24 h did not normalize the high T3 concentrations, suggesting that high T3 is a powerful counterregulatory mechanism activated following sleep deprivation. Modern life has shortened sleep time, and the consequences of sleep deprivation have been examined in both human subjects and animal models. As the relationship between thyroid function and sleep deprivation has not been fully investigated, the aim of this study was to assess the hypothalamic-pituitary-thyroid axis and thyroid hormone metabolism following paradoxical sleep deprivation (PSD) and sleep restriction (SR) in rats. The effects of a 24 h rebound period were also studied. Male Wistar rats (200-250 g, n = 10 per group) were subjected to sleep deprivation via the modified multiple platform method. Rats were assigned to the following seven groups: control, PSD for 24 or 96 h, 24 or 96 h of sleep deprivation with rebound (PSD24R and PSD96R), SR for 21 days (SR21) and SR21 with rebound (SR21R). Blood samples were collected to determine the 3,5,3'-triiodothyronine (T3 ), thyroxine (T4 ) and thyroid-stimulating hormone concentrations. Brown adipose tissue iodothyronine deiodinase type 2 (D2) activity was also evaluated. Body weight gain was dramatically reduced (by ∼50-100%) in all sleep-deprived and sleep-restricted rats; rebound restored this parameter in only the PSD24R group. The serum TSH and T4 concentrations decreased, whereas T3 increased in both the PSD24 and PSD96 groups compared with control animals (P < 0.05). Only PSD24R and PSD96R normalized T4 and thyroid-stimulating hormone concentrations, respectively, independently of the higher circulating T3 concentrations (∼20-30%) noted in all groups compared with control animals (P < 0.05). Brown adipose tissue D2 activity increased in the PSD 24 and 96 h groups (∼10 times), and PSD24R was more effective than PSD96R at restoring basal brown adipose tissue D2 activity. Our data suggest that thyroid hormone metabolism adapts to sleep deprivation-induced hypothalamic-pituitary-thyroid alterations and increases T4 to T3 activation peripherally, thereby increasing circulating T3 in rats.

The FoxO3/type 2 deiodinase pathway is required for normal mouse myogenesis and muscle regeneration

The active thyroid hormone 3,5,3' triiodothyronine (T3) is a major regulator of skeletal muscle function. The deiodinase family of enzymes controls the tissue-specific activation and inactivation of the prohormone thyroxine (T4). Here we show that type 2 deiodinase (D2) is essential for normal mouse myogenesis and muscle regeneration. Indeed, D2-mediated increases in T3 were essential for the enhanced transcription of myogenic differentiation 1 (MyoD) and for execution of the myogenic program. Conversely, the expression of T3-dependent genes was reduced and after injury regeneration markedly delayed in muscles of mice null for the gene encoding D2 (Dio2), despite normal circulating T3 concentrations. Forkhead box O3 (FoxO3) was identified as a key molecule inducing D2 expression and thereby increasing intracellular T3 production. Accordingly, FoxO3-depleted primary myoblasts also had a differentiation deficit that could be rescued by high levels of T3. In conclusion, the FoxO3/D2 pathway selectively enhances intracellular active thyroid hormone concentrations in muscle, providing a striking example of how a circulating hormone can be tissue-specifically activated to influence development locally.

Inhibition of pituitary type 2 deiodinase by reverse triiodothyronine does not alter thyroxine-induced inhibition of thyrotropin secretion in hypothyroid rats

Intrapituitary triiodothyronine (T3) production plays a pivotal role in the control of TSH secretion. Its production is increased in the presence of decreased serum thyroxine (T4) concentrations and the enzyme responsible, deiodinase type 2 (D2), is highest in hypothyroidism. In order to document the role of this enzyme in adult rats we developed an experimental model that inhibited this enzyme using the specific inhibitor, reverse T3 (rT3). Hypothyroidism was induced with propylthiouracil (PTU; 0.025 g/l in drinking water) which in addition blocked deiodinase type 1 (D1) activity, responsible for the rapid clearance of rT3 in vivo. During the last 7 days of the experiment, the hypothyroid rats were injected (s.c.) for 4 days with 0.4 or 0.8 nmol T4 per 100 g body weight (bw) per day. For the last 3 days, the same amount of T4 was infused via s.c. minipumps. In additional groups, 25 nmol rT3/100 g bw per day were added to the 3-day infusion of T4. Infusion of 0.4 nmol T4/100 g bw per day did not affect the high serum TSH levels, 0.8 nmol T4/100 g bw per day decreased them to 57% of the hypothyroid values. The infusions of rT3 inhibited D2 activity in all organs where it was measured: the pituitary, brain cortex and brown adipose tissue (BAT). In the pituitary, the activity was 27%, to less than 15% of the activity in hypothyroidism. Despite that, serum TSH levels did not increase, serum T4 concentrations did not change and the changes in serum T3 were minimal. We conclude that in partly hypothyroid rats, a 3-day inhibition of D2 activity, without concomitant change in serum T4 and minimal changes in serum T3 levels, is not able to upregulate TSH secretion and we postulate that this may be a reflection of absent or only minimal changes in circulating T3 concentrations.

Triiodothyronine Down-Regulates Thyrotropin-Releasing Hormone (TRH) Synthesis and Decreases pTRH-(160-169) and Insulin Releases from Fetal Rat Islets in Culture

TRH is localized with insulin in β-cells. It is synthesized as a prohormone containing five copies of TRH and seven cryptic peptides, including pro (p)-TRH-(160–169). Thyroid hormone regulates the expression of several genes encoding peptide hormones. We found that circulating T3 concentrations were inversely correlated with TRH levels in two physiopathological situations. There are low concentrations of circulating thyroid hormone and very high concentrations of TRH and pTRH-(160–169) during development, and experimental hypothyroidism results in higher concentrations of prepro (pp)-TRH messenger RNA (mRNA) and TRH content in the adult rat pancreas than are present in the euthyroid pancreas. The present study was carried out to investigate the interaction between T3 and pancreatic TRH during the functional maturation of islets in culture and to validate the data obtained in vivo. T3 decreases ppTRH mRNA in islets in a dose-dependent manner. The primary impact of T3 on islet function may be mediated by ppTRH mRNA, as short term T3 treatment had no effect. Long term T3 treatment reduced the islet TRH content and the amounts of pTRH-(160–169) and insulin released. This secretory pattern and coordinated regulation of pTRH-(160–169) and insulin suggests that pTRH-(160–169) plays a specific role in the regulation of insulin secretion.

Triiodothyronine down-regulates thyrotropin-releasing hormone (TRH) synthesis and decreases pTRH-(160-169) and insulin releases from fetal rat islets in culture.

TRH is localized with insulin in beta-cells. It is synthesized as a prohormone containing five copies of TRH and seven cryptic peptides, including pro (p)-TRH-(160-169). Thyroid hormone regulates the expression of several genes encoding peptide hormones. We found that circulating T3 concentrations were inversely correlated with TRH levels in two physiopathological situations. There are low concentrations of circulating thyroid hormone and very high concentrations of TRH and pTRH-(160-169) during development, and experimental hypothyroidism results in higher concentrations of prepro (pp)-TRH messenger RNA (mRNA) and TRH content in the adult rat pancreas than are present in the euthyroid pancreas. The present study was carried out to investigate the interaction between T3 and pancreatic TRH during the functional maturation of islets in culture and to validate the data obtained in vivo. T3 decreases ppTRH mRNA in islets in a dose-dependent manner. The primary impact of T3 on islet function may be mediated by ppTRH mRNA, as short term T3 treatment had no effect. Long term T3 treatment reduced the islet TRH content and the amounts of pTRH-(160-169) and insulin released. This secretory pattern and coordinated regulation of pTRH-(160-169) and insulin suggests that pTRH-(160-169) plays a specific role in the regulation of insulin secretion.

Expression and function of thyroid hormone receptor variants in normal and chronically diseased human liver.

As the liver represents a major target organ for thyroid hormone action, we compared the expression of thyroid hormone receptor (TR) alpha and beta variants in normal human liver and liver affected by primary biliary cirrhosis, sclerosing cholangitis, cryptogenic cirrhosis, and alcoholic cirrhosis (n = 6 in each group). Western blot analysis using specific polyclonal antibodies to alpha 1 or beta 1 TRs or to the related non-T3-binding c-erbA alpha 2 variant revealed abundant expression of TRs in normal and diseased liver, with no difference in size or abundance of TR proteins. Immunocytochemistry likewise revealed abundant nuclear expression of TR proteins in normal and diseased liver, with similar patterns and intensity of staining. Despite abundant TR protein expression, Northern blot hybridization of polyadenylated ribonucleic acid (RNA; 10 micrograms) to TR complementary DNAs revealed only a weak signal for c-erbA alpha 2 messenger RNA (mRNA). Comparison of the level of expression of the thyroid hormone-regulated mRNAs encoding T4-binding globulin, sex hormone-binding globulin, cortisol-binding globulin, and transthyretin in normal and diseased tissue revealed no significant difference, suggesting that hepatocellular expression of these mRNAs is maintained in chronic liver disease despite a marked reduction in circulating T3 concentrations.

Down-regulation of thyroxine-binding globulin messenger ribonucleic acid by 3,5,3'-triiodothyronine in human hepatoblastoma cells.

A sensitive [125I]-T4 binding assay was used to measure serum T4-binding globulin (TBG) in 60 individuals selected on the basis of their total circulating T3 concentrations, and a relationship between TBG and circulating thyroid hormone levels in humans was confirmed. There was a significant correlation between serum TBG and T3 or free T4 index. TBG secretion and TBG messenger ribonucleic acid (mRNA) production were studied with a continuous culture of the human hepatoblastoma cell line, HepG2. Cells were maintained in serum-free media for experimental manipulations. The addition of 100 nmol/L T3 to the cell medium resulted in a time-dependent down-regulation of TBG mRNA to 33 +/- 6% (+/- SD, n = 4) of untreated control levels by 24 h. Suppression of TBG mRNA was first detectable at 8 h (57% of untreated control levels). The effect of T3 was dose-responsive, with half-maximal suppression of TBG mRNA occurring at a bioavailable T3 concentration of approximately 30 pmol/L. The effect of T3 on TBG mRNA was not caused by a change in mRNA stability. Proteins secreted by HepG2 cells bound T4 with an affinity identical to that of normal circulating TBG. Cell secretion of TBG was parallel to total protein secretion and consistent with a TBG secretion rate of 50 ng/10(6) cells per day. Variations in the concentration of secreted binding protein in the presence of T3 corresponded to the changes observed in TBG mRNA. These data show that circulating TBG concentration is negatively correlated with total serum T3 in vivo. The corresponding down-regulation observed between TBG mRNA and secreted protein in HepG2 cells suggests that this effect is the result of the action of T3 on cellular TBG mRNA synthesis.

The thyroid gland is a major source of circulating T3 in the rat.

In rats, the respective contribution of the thyroid and peripheral tissues to the pool of T3 remains unclear. Most, if not all, of the circulating T3 produced by extrathyroidal sources is generated by 5'-deiodination of T4, catalyzed by the selenoenzyme, type I iodothyronine 5'-deiodinase (5'D-I). 5'D-I in the liver and kidney is almost completely lost in selenium deficiency, resulting in a marked decrease in T4 deiodination and an increase in circulating T4 levels. Surprisingly, circulating T3 levels are only marginally decreased by selenium deficiency. In this study, we used selenium deficiency and thyroidectomy to determine the relative contribution of thyroidal and extrathyroidal sources to the total body pool of T3. Despite maintaining normal serum T4 concentrations in thyroidectomized rats by T4 replacement, serum T3 concentrations remained 55% lower than those seen in intact rats. In intact rats, restricting selenium intake had no effect on circulating T3 concentrations. Decreasing 5'D-I activity in the liver and kidney by > 90% by restricting selenium intake resulted in a further 20% decrease in serum T3 concentrations in the thyroidectomized, T4 replaced rats, suggesting that peripheral T4 to T3 conversion in these tissues generates approximately 20% of the circulating T3 concentrations. While dietary selenium restriction markedly decreased intrahepatic selenium content (> 95%), intrathyroidal selenium content decreased by only 27%. Further, thyroid 5'D-I activity actually increased 25% in the selenium deficient rats, suggesting the continued synthesis of this selenoenzyme over selenoproteins in other tissues in selenium deficiency. These data demonstrate that the thyroid is the major source of T3 in the rat and suggest that intrathyroidal T4 to T3 conversion may account for most of the T3 released by the thyroid.

Feeding of rapeseed presscake meal to pigs: effects on thyroid morphology and function and on thyroid hormone blood levels, on liver and on growth performance.

Effects of feeding soybean meal (SBM) or a mixture of SBM and 5%, 10% and 15% of either 0- and 00-rapeseed presscake meal (RPM) on thyroid, liver and growth performance were studied in castrated boars. Goitrin (G) intake was closely correlated with total glucosinolate intake and blood thiocyanate concentration. In pigs fed 10% and 15% 0-RPM refused feed was enhanced and daily gain decreased. Liver weight and DNA/total liver increased with increasing G intake. Thyroid weights increased up to a G intake of 4.6 mmol/d and then decreased with higher G intake. DNA content/g dry matter (DM) increased continuously and total thyroidal DNA content up to a G intake of 4.6 mmol/l, indicating hyperplasia. Epithelial thickness increased up to a G intake of 13.3 mmol/d, whereas follicular area only up to an intake of 4.6 mmol G/d and then decreased, mirrored by a decrease of DM content. Thyroidal thyroxine (T4) and triiodothyronine (T3) content decreased with increasing G intake. Blood serum levels of T4 decreased with increasing G intake, whereas T3 levels only tended to decrease. Therefore, measured parameters differently reacted to different G intake. In groups fed high amounts of G, hyperplasia was not sufficient to maintain physiological circulating T4 levels, but enhanced thyroidal secretion T3 and/or extrathyreoidal T4 to T3 conversion obviously allowed maintenance of near normal circulating T3 concentrations and barely reduced growth rate despite marked hypothyroidism.

Effects of selenium deficiency on thyroid hormone economy in rats.

In selenium-deficient rats, peripheral T4 to T3 conversion is markedly decreased due to the loss of the selenoprotein, type I iodothyronine 5'-deiodinase (5'D-I). Despite the marked increase in circulating T4 that results from this loss of 5'D-I, serum T3 concentrations in selenium-deficient rats remain in the normal range. To determine the physiological mechanism(s) that maintains circulating T3 when peripheral T4 to T3 conversion is impaired, we examined the interrelationships between selenium intake and the metabolism of T3 and T4 in the rat. In euthyroid rats, selenium deficiency caused the expected loss of 5'D-I, with a 52% increase in serum T4, which paralleled an increase in the T4 biological half-life. Consistent with the prolonged t1/2 of T4, short term thyroidectomy (48 h) in selenium-deficient rats failed to decrease serum T4 concentrations to the levels observed in short term thyroidectomized, selenium-supplemented rats. Short term thyroidectomy also caused an expected 33% decrease in liver 5'D-I and a 44% increase in brain type II iodothyronine 5'-deiodinase (5'D-II) activities in selenium-supplemented rats. However, in selenium-deficient rats, short term thyroidectomy did not affect 5'D-I or 5'D-II activities. In contrast to the selenium-dependent changes in circulating T4 levels, little or no change in circulating T3 concentrations occurred. There was a 20% increase in the T3 half-life in selenium-deficient rats. The serum T3 sulfate concentration was increased, and T3 deiodination was reciprocally decreased in the selenium-deficient rats. These data suggest that increased T3 sulfate generation in selenium-deficient rats may lead to greater T3 availability through enterohepatic recycling of the iodothyronine and may explain why there are only minor changes in serum T3 concentrations in selenium-deficient rats.

Participation of tri-iodothyronine and metabolic clearance rate in the inhibition of growth hormone secretion in thyroxine-treated domestic fowl

Surgical thyroidectomy increases basal and TRH-induced GH concentrations in the peripheral plasma of immature domestic fowl. Replacement therapy with thyroxine (T4; 100 micrograms/kg per day for 7 days, i.m.) suppressed the GH responses to thyroidectomy. Bolus administration of T4 (10 micrograms/kg, i.m.) to thyroidectomized birds promptly lowered the circulating GH concentrations, which remained suppressed for at least 4 h. Chronic (daily injections for 7 days) or acute (one injection) pretreatment of thyroidectomized birds with iopanoic acid (IOP; 40 mg/bird, i.m.) before the bolus administration of T4 attenuated, but did not prevent, inhibition of circulating GH levels by T4. Administration of IOP (40 mg/bird i.m.) 24 h and immediately before the administration of tri-iodothyronine (T3; 3 micrograms/kg, i.m.) or T4 (10 micrograms/kg, i.m.) also failed to suppress thyroidal inhibition of circulating GH concentrations in thyroidectomized birds. Administration of IOP alone had no effect on GH concentrations. Circulating T3 concentrations were not enhanced following the administration of T4 to IOP-treated birds, indicating its inhibition of hepatic monodeiodinase activity. The metabolic clearance rate (MCR) of 125I-labelled chicken GH in the plasma of thyroidectomized fowl was less than that in sham-thyroidectomized birds. Following pretreatment with T4 (100 micrograms/kg per day for 7 days) sham-thyroidectomized and thyroidectomized birds did not differ significantly in their MCR. The GH secretion rate in thyroidectomized birds was similar to that in sham-thyroidectomized birds and in both groups was markedly reduced following pretreatment with T4. These results demonstrate thyroidal inhibition of circulating GH concentrations in fowl.(ABSTRACT TRUNCATED AT 250 WORDS)

INHIBITION OF CONVERSION OF THYROXINE TO TRIIODOTHYRONINE IN PATIENTS WITH SEVERE CHRONIC ILLNESS

Many clinically euthyroid patients with severe, chronic, non-throidal illnesses (i.e. sick euthyroid patients) have very low circulating concentrations of total and absolute free triiodothyronine (T3), low-normal concentrations of total thyroxine (T4), elevated concentrations of absolute free T4, and circulating concentrations of thyrotrophin (TSH) that are either normal or subnormal. This study was undertaken to elucidate the mechanism of the low circulating T3 concentrations. The disappearance rate of 125 I-T3 from the circulation of five representative sick euthyroid patients was studied and found to be slower, but not significantly so, compared with three control subjects, thus excluding an increased destruction rate as the cause of the low T3 levels. A selective decrease of T3 secretion from the thyroid gland of these patients was also excluded by the results of TSH stimulation tests. Inhibition of extra-thyroidal conversion of T4 to T3 was suggested by studies of the thyroid function in a hypothyroid woman with a Grade IV lymphoma on T4 replacement therapy. When the lymphoma was in remission, her circulating T3 concentration was 2-55 nmol/l but when it relapsed it fell to 0-55 nmol/l. The T4 concentrations were 124-7 nmol/l and 126 nmol/1 respectively. Decreased monodeiodination of T4 to T3 in sick euthyroid patients was confirmed by paper chromatography of extracted serum obtained 48 h after an i.v. injection of 125 I-T4 into two severely ill patients from the intensive therapy unit and a control subject. Peaks of radioactivity corresponding to 125 I-T4 and 125 I-T3 were detected in the control subject, but only a single peak corresponding to 125 I-T4 was detected in the ill patients.