The effect of propylthiouracil on thyroid-stimulating hormone-induced alterations in iodothyronine secretion from perfused dog thyroids

The aim of this study was to see whether the inhibitory effect of propylthiouracil on thyroidal secretion of 3,5,3'-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3) could be reproduced in intensively stimulated thyroids, and to elucidate whether an increase in the fractional deiodination of thyroxine (T4) to T3 and rT3 during iodothyronine secretion might be responsible for the transient fall in the T4/T3 and T4/rT3 ratios in thyroid secretion seen in the early phase after stimulation of thyroid secretion. For this purpose T4, T3 and rT3 were measured in effluent from isolated dog thyroid lobes perfused in a non-recirculation system using a synthetic hormone free medium. 1 mmol/1 propylthiouracil induced a significant reduction in thyroid-stimulating hormone (TSH) stimulated T3 and rT3 release while the release of T4 was unaffected. This supports our previous conclusion that T4 is partially monodeiodinated to T3 and rT3 during thyroid secretion. Infusion of 1 mmol/l propylthiouracil for 30 min or 3 mmol/l propylthiouracil for 120 min did not abolish the transient fall in effluent T4/T3 and T4/rT3 induced by TSH stimulation. Thus, this phenomenon seems not to depend on intrathyroidal iodothyronine deiodinating processes.     

Different pathways of iodothyronine 5′-deiodination in rat cerebral cortex

Microsomal fractions of rat cerebral cortex catalyze the 5′-deiodination of 3,3′,5′-triiodothyronine (rT₃) in the presence of thiols such as dithiothreitol. Evidence is presented that two different enzymatic pathways are involved. One of these has a low apparent K_m (2.7 nM) for rT₃, is inhibited by nanomolar concentrations of thyroxine (T₄), but not by up to 1 mM 6-propyl-2-thiouracil (PTU). The other pathway has a high apparent K_m (31 nM) for rT₃, is inhibited by PTU, but not by <1 μM T₄. The relative proportion of rT₃ 5′-deiodination via either pathway depends on thyroid status, with increased contributions from the low-K_m system especially in short-term hypothyroidism.     

Circulating reverse triiodothyronine (rT3) in the dog

Plasma reverse triiodothyronine (rT₃) concentration was measured by radio immunoassay (RIA) in a group of 15 dogs. The mean rT₃ concentration was 187 ng/100 ml which was 3 times higher than radioimmunoassayable triiodothyronine (T₃) concentration. rT₃ measurement in thyroid and peripheral venous plasma in 3 dogs showed that the unusually high circulating rT₃ levels in this species could not be explained on the basis of augmented thyroidal rT₃ secretion. Study of rT₃-protein binding by equilibrium dialysis also failed to show any evidence of unusual rT₃-protein interaction (rT₃ free fraction was 2 — 3 times greater than in normal human serum). Among all the species examined so far (man, monkey, sheep, dog and rat), only in the dog are the circulating rT₃ levels significantly higher than T₃ suggesting that in this species the 5-deiodination, in marked contrast to the 5'-deiodination noted in several other species, is a major pathway normally involved in the initial monodeiodination of T₄.     

Serum principal iodothyronines and the influence of cold exposure associated with shearing in the sheep

The effect of cold exposure caused by shearing on serum thyroid hormone (TH) concentrations in sheep kept at an ambient temperature of 8.5°C was studied. While the deep body temperature fell to the lowest level 4 h after shearing the concentration of triiodothyronine (T₃) increased to a peak value at that time. Thyroxine (T₄) and metabolically inactive reverse triiodothyronine (rT₃) levels reached their peak value after 24 h. The $\text{T}{}_3\text{T}{}_4$ ratio reached a maximum at about 4 h and $\text{rT}{}_3\text{T}{}_4$ and $\text{rT}{}_3\text{T}{}_3$ ratios rose to maximum values about 24 h after shearing. This sequence of events suggest a biphasic response to cold—an immediate secretion of TH from the thyroid gland, followed by adaptive alteration in T₃ and rT₃ generation in the extrathyroidal tissues.     

Rapid Responses to Reverse T3 Hormone in Immature Rat Sertoli Cells: Calcium Uptake and Exocytosis Mediated by Integrin

There is increasing experimental evidence of the nongenomic action of thyroid hormones mediated by receptors located in the plasma membrane or inside cells. The aim of this work was to characterize the reverse T₃ (rT₃) action on calcium uptake and its involvement in immature rat Sertoli cell secretion. The results presented herein show that very low concentrations of rT₃ are able to increase calcium uptake after 1 min of exposure. The implication of T-type voltage-dependent calcium channels and chloride channels in the effect of rT₃ was evidenced using flunarizine and 9-anthracene, respectively. Also, the rT₃-induced calcium uptake was blocked in the presence of the RGD peptide (an inhibitor of integrin-ligand interactions). Therefore, our findings suggest that calcium uptake stimulated by rT₃ may be mediated by integrin α_vβ₃. In addition, it was demonstrated that calcium uptake stimulated by rT₃ is PKC and ERK-dependent. Furthermore, the outcomes indicate that rT₃ also stimulates cellular secretion since the cells manifested a loss of fluorescence after 4 min incubation, indicating an exocytic quinacrine release that seems to be mediated by the integrin receptor. These findings indicate that rT₃ modulates the calcium entry and cellular secretion, which might play a role in the regulation of a plethora of intracellular processes involved in male reproductive physiology.     

The crystal structure of a major metabolite of thyroid hormone: 3,3′,5′-triiodo-L-thyronine

Two independent conformations of the thyroinactive thyroid hormone metabolite, 3,3′,5′-triido-L-thyronine (rT₃) were determined by X-ray diffraction methods. The conformations show significant difference in the lettering geometry when compared with those of the thyroactive thyroxine (T₄) and 3,5,3′-triido-L-thyronine (T₃). The diphenyl ether conformation of the two conformers of rT₃ is an anti-skewed one, in which the torsion angels, φ (C5-C4-O4-Cl′) are 8° and ?6°, and φ′ are 86° and 87°. This conformation is in contrast to a twist-skewed one of T₄ and T₃. The difference in the binding abilities between T₄, T₃ and rT₃ to thyroxine binding carrier proteins in serum or to a nuclear receptor protein may be explained by the characteristics solid-state conformations of these metabolites.     

Regulation of thyroid hormone metabolism in rat liver fractions

The nature of the conversion of thyroxine (T₄) to triiodothyronine (T₃) and reverse triiodothyronine (rT₃) was investigated in rat liver homogenate and microsomes. A 6-fold rise of T₃ and 2.5-fold rise of rT₃ levels determined by specific radioimmunoassays was observed over 6 h after the addition of T₄. An enzymic process is suggested that converts T₄ to T₃ and rT₃. For T₃ the optimal pH is 6 and for rT₃, 9.5. The converting activity for both T₃ and rT₃ is temperature dependent and can be suppressed by heat, H₂O₂, merthiolate and by 5-propyl-2-thiouracil. rT₃ and to a lesser degree iodide, were able to inhibit the production of T₃ in a dose related fashion. Therefore the pH dependendy, rT₃ and iodide may regulate the availability of T₃ or rT₃ depending on the metabolic requirements of thyroid hormones.     

Effect of insulin-induced hypoglycemia on the serum concentrations of thyroxine,triiodothyronine and reverse triiodothyronine

The effect of insulin-induced hypoglycemia on serum thyroid hormone concentrations was studied in nine healthy individuals. Before, during and after the hypoglycemia blood samples were taken for measurement of the concentrations of glucose, thyroxine (T₄), triiodothyronine (T₃), reverse triiodothyronine (rT₃), catecholamines and pituitary hormones.There was no change in the mean serum T₄ level (± the standard error of the mean) of 67 ± 2 μg/l. However, the T₃ concentrations rose from a mean basal level of 1.86 ± 0.06 μg/l to a mean peak of 2.51 ± 0.21 μg/l (P < 0.01) at 45 minutes after the insulin injection, and the rT₃ concentrations fell from a mean basal level of 0.184 ± 0.008 μg/l to a mean nadir of 0.171 ± 0.022 μg/l (not a significant change). The mean peak epinephrine level was 545 ± 103 ng/l and it occurred between 30 and 45 minutes after the insulin injection; the mean peak norepinephrine level was 584 ± 114 ng/l and it occurred between 30 and 90 minutes after the injection. The growth hormone levels reached a mean peak of 26.1 ± 4.8 μg/l and the plasma cortisol levels rose to 215 ± 9 μg/l. The mean basal prolactin level was 8.5 ± 0.9 μg/l; in five subjects there was a rise to a mean peak of 50.6 ± 14.6 μg/l, whereas in the remaining four no significant increase occurred. No correlation was found between the changes in the serum T₃ concentration and any of the other factors studied.It was concluded that acute hypoglycemia is associated with a rapid increase in the serum T₃ concentration.     

Phenolic ring deiodination in cultured rat hepatoma cells, and subcellular localization of deiodinases in cultured rat hepatoma, monkey hepatocarcinoma cells and normal rat liver homogenates

The cultured rat hepatoma cell (R117-21B) homogenates metabolized 3,[3′,5′-¹²⁵I]triiodothyronine by phenolic ring deiodination and produced radioactive iodide and 3,3′-diiodothyronine. Thyroxine (T₄) was converted to 3,3′,5-triidothyronine (T₃). The production of ¹²⁵I⁻ presented the deiodinase activity. The optimal pH for phenolic ring deiodination was observed to be pH 6.0–7.0. This enzyme reaction was accelerated by dithiothreitol. Propylthiouracil strongly inhibited the phenolic ring deiodination at 0.1 mM, whereas an effect of 20 mM methylmercaptoimidazol on the deiodination was very weak or absent.Excess unlabeled iodothyronines (T₄, T₃ and 3,5-diiodo-l-thyronine inhibited the phenolic ring deiodination of labeled 3,3′,5′-triiodothyronine, althought their inhibitory effect was slightly different. Triiodothyroacetic acid was a better inhibitor than T₃. Diiodotyrosine did not affect phenolic ring deiodination in cultured rat hepatoma cell homogenates.Phenolic and nonphenolic ring deiodinase activities of cultured monkey hepatocarcinoma cell and rat liver homogenates were also studied by the use of 3,[3′,5′-¹²⁵I]triiodothyronine and [3,5-¹²⁵I]thyroxine, respectively. Both deiodinase activities were observed in particulate fractions (mitochondrial and microsomal) of cultured cell and rat liver homogenates.     

Thyroid function and thyrotropin levels in rabbits immunized to produce antibodies against thyroid hormones

Various parameters of thyroid function were studied in 27 rabbits, out of which 10 were immunized to produce antibodies against triiodothyronine (T₃), 9 against thyroxine (T₄) and 8 were normals. Estimations of T₃, T₄, Free T₄ (FT₄) and thyrotropin (TSH) in blood, qualitative and quantitative analysis of iodoamino acids in serum, protein bound iodine-131 (PB¹³¹I), butanol extractable iodine-125 (BE¹²⁵I) and measurement of the disappearance rates of ¹²⁵I-labelled T₃ and T₄ from plasma were done. In addition, glandular changes were also studied by measurement of ¹³¹I uptake, thyroid scanning and chromatographic analysis of hydrolysate of soluble iodoproteins. In T₃ immunized animals, levels of T₃ in serum increased by 38 to 125 times, levels of TSH also showed a significant rise (7.4 ± 1.2 vs 28 ± 9 ng/mL). Chromatographic analysis of iodoamino acids in serum as well as in the hydrolysate of the thyroid gland demonstrated a selective increase in synthesis of T₃. Rate of disappearance of T₃ from blood showed a significant decline. Thyroid glands in the immunized rabbits showed signs of hypertrophy and hyperplasia. Identical studies done in rabbits immunized to produce antibodies against T₄ showed a similar pattern though of variable degree. Our studies indicate that the thyroid glands of the immunized rabbits undergo marked alterations resulting in selective increase in the synthesis and secretion of the particular thyroid hormone against which they were immunized. They do so under the influence of increased levels of TSH.     

Serum T4, T3 and reverse T3 in ethanol fed rats.

To investigate the influence of chronic ethanol consumption on circulating thyroid hormone levels, male and female rats were given 20% ethanol as the only drinking solution daily for 8 weeks. Blood ethanol levels ranged 30–45 mg/L. In male rats serum T₄ decreased from the initial mean ± SD value of 5.2±1.4 to3.0 ±0.7 μg/dl; T₃ decreased from initial value of 97±14 to 66±11 ng/dl and rT₃ decreased from initial value of 19±9 to 10±1 ng/dl after 8 weeks of ethanol ingestion. Under similar experimental conditions, female rats showed a significant decrease in serum T₄ and rT₃ levels; however, T₃ levels decreased slightly but not significantly as compared to initial values. The results indicate adverse effect of chronic ethanol intake on serum thyroid hormone levels in rats.     

Absence of A Thyroid Phenotype in Sortilin-Deficient Mice

Objective: The Vps10p family member sortilin is expressed in thyroid epithelial cells where it contributes to recycling of the thyroid hormone precursor thyroglobulin (Tg), a process that is thought to render hormone release more effective. Here we investigated the functional impact of sortilin in the thyroid gland using sortilin-deficient mice.Methods: We measured free T₄, thyroid-stimulating hormone (TSH) and Tg serum levels and studied thyroid morphology in 14 sortilin-deficient (Sort1)^-/-and 12 wildtype (WT) mice.Results: Serum free T₄ levels did not differ between Sort1^-/-and WT females but were significantly lower in Sort1^-/-males compared with WT (P = .0424). Neither serum TSH nor Tg levels differed between Sort1^-/-and WT mice, regardless of sex. On the same line, no thyroid histology differences were observed.Conclusion: Our findings seem to exclude a role of sortilin in thyroid hormone secretion, although it is possible that the absence of sortilin may result in a thyroid phenotype if combined with other molecular defects of thyroid hormone synthesis and secretion or under iodine deficiency.Abbreviations: T₄ = thyroxine Sort1 = Sortilin 1 Tg = thyroglobulin TSH = thyroid-stimulating hormone WT = wild type     

Thyroid hormone analogues potentiate the antiviral action of interferon-γ by two mechanisms

L-thyroxine (L-T₄) potentiates the antiviral activity of human interferon-γ (IFN-γ) in HeLa cells. We have added thyroid hormone and analogues to cells either 1) for 24 h pretreatment prior to 24 h of IFN-γ (1.0 IU/ml), 2) for 24 h cotreatment with IFN-γ, 3) for 4 h, after 20 h cell incubation with IFN-γ, alone, or 4) for 24 h pretreatment and 24 h cotreatment with IFN-γ. The antiviral effect of IFN-γ was then assayed. L-T₄ potentiated the antiviral action of IFN-γ by a reduction in virus yield of more than two logs, the equivalent of a more than 100-fold potentiation of the IFN's antiviral effect. 3,3′,5-L-triiodothyronine (L-T₃) was as effective as L-T₄ when coincubated for 24 h with IFN-γ but was less effective than L-T₄ when coincubated for only 4 h. D-T₄, D-T₃, 3,3′,5-triiodothyroacetic acid (triac), tetraiodothyroacetic acid (tetrac), and 3,5-diiodothyronine (T₂) were inactive. When preincubated with L-T₄ for 24 h prior to IFN-γ treatment, tetrac blocked L-T₄ potentiation, but, when coincubated with L-T₄ for 4 h after 20 h IFN-γ, tetrac did not inhibit the L-T₄ effect. 3,3′,5′-L-triiodothyronine (rT₃) also potentiated the antiviral action of IFN-γ, but only in the preincubation model. Furthermore, the effects of rT₃ preincubation and L-T₃ coincubation were additive, resulting in 100-fold potentiation of the IFN-γ effect. When L-T₄, L-T₃, or rT₃, plus cycloheximide (5 μg/ml), was added to cells for 24 h and then removed prior to 24 h IFN-γ exposure, the potentiating effect of the three iodothyronines was completely inhibited. In contrast, IFN-γ potentiation by 4 h of L-T₄ or L-T₃ coincubation was not inhibited by cycloheximide (25 μg/ml). These studies demonstrate two mechanisms by which thyroid hormone can potentiate IFN-γ's effect: 1) a protein synthesis-dependent mechanism evidenced by enhancement of IFN-γ's antiviral action by L-T₄, L-T₃, or rT₃ preincubation, and inhibition of enhancement by tetrac and cycloheximide, and 2) a protein synthesis-independent (posttranslational) mechanism, not inhibited by tetrac or cycloheximide, demonstrated by 4 h coincubation of L-T₄ or L-T₃, but not rT₃, with IFN-γ. The protein synthesis-dependent pathway is responsive to rT₃, a thyroid hormone analogue generally thought to have little effect on protein synthesis. A posttranslational mechanism by which the antiviral action of IFN-γ can be regulated has not previously been described. © 1996 Wiley-Liss, Inc.     

Effects of ovine prolactin on iodide metabolism and thyroid activity in the eel

Summary In the eel, ovine prolactin (oPrl) treatment (0.018 IU/day·g body weight), for 8 to 13 days modifies neither iodide absorption from the water nor excretion, extrathyroidal metabolism and plasma level of iodide.Thyroid activity, evaluated by epithelial cell height, radioiodine uptake and absolute iodide uptake is approximately twice that of controls. However, the amounts of total iodine, thyroxine (T₄) and triiodothyronine (T₃) in thyroid are unaltered by oPrl. Therefore, the decrease of plasma T₄ and the increase of plasma T₃, previously observed in oPrl-treated eels, do not result from a preferential thyroidal secretion of T₃, but only from a stimulation of peripheral conversion of T₄ to T₃. Furthermore, the increased thyroid activity probably originates from a decreased feedback inhibition following the fall of circulating T₄ induced by oPrl.Abbreviations oPrl ovine prolactin - T ₄ Thyroxine - T ₃ 3.5.3 triiodothyronine - TRH thyrotropin releasing hormone - TSH thyroid stimulating hormone - PBI protein bound iodine     

A role for intracellular calcium and calmodulin in the release of triiodothyronine from human thyroid-cell monolayer cultures

Human thyroid cells in monolayer responded to acute stimulation by TSH with an increase in the secretion of T₃. This process appeared to be dependent on a rise in the cytosolic calcium concentration since the antagonist of intraceliular calcium mobilization, TMB-8, was found to inhibit the release of T₃ in response to TSH. The importance of intracellular calcium was further shown using the agent veratridine which increases the free calcium level within cells; veratridine potentiated the stimulation of T₃ secretion by TSH and itself stimulated the release of T₃ to a level higher than that seen in the presence of TSH alone. The calcium ionophore A23197 produced a biphasic effect on T₃ secretion from human thyroid monolayers; at low concentrations, A23187 caused a decrease in both unstimulated and TSH-stimulated T₃ secretion but above a concentration of 1 M, T₃ secretion was increased. The calmodulin antagonist W7 was found to inhibit T₃ release in response to TSH, indicating a role for calmodulin in mediating the effects of intracellular calcium on T₃ secretion.     

Neurobiology and pharmacology of Huntington's disease.

Reverse triiodothyronine (rT₃) which is apparently a product of 5-monodeiodination of thyroxine (T₄) at the periphery, was measured in 199 chickens of various strains. rT₃ was virtually absent in young birds (less than 1 week to 8 weeks of age) in marked contrast to the elevated rT₃ levels found in human and other mammalian neonates. At one to two years of age there was a significant increase in the number of birds with detectable rT₃. However, rT₃ concentrations were low and often close to the detection limits of the assay in contrast to significant rT₃ levels found in mammals (man, monkey, sheep, rat and dog). An apparent sex difference in relation to rT₃ formation was noted; 46.4% of 97 females and 9.3% of 54 males showed detectable rT₃ levels. The observations described suggest a species difference in regard to peripheral T₄ monodeiodination between birds and mammals.     

Genetics of heat tolerance and thyroid function in Athens-Canadian randombred chickens

Summary Five experiments were conducted to assess the genetic variation in thyroid function (T₃, T₄), body weight and heat stress survival time in chickens. Thyroxine (T₄) levels were found to be elevated in response to 4 and 8 g bovine thyroid stimulating hormone (TSH) in experiment I. In experiment II, 4 g of TSH was injected into chickens from 30 sire families of the Athens-Canadian Randombred population. The heritability of T₄ levels after TSH injection was high. In experiment III, families identified as having innate high or low T₄ levels after TSH injection and a group of control birds were subjected to a heat Stressor of 50 °C for up to 240 min at six weeks of age and heat stress survival time was studied. The groups did not differ from each other in heat stress survival time. Experiment IV was similar to experiment I except triiodothyronine (T₃) was also measured after TSH injection. Both T₄ and T₃ levels after TSH injection were moderately heritable. In experiment V birds were reared to six weeks of age and heritability calculated for body weight, T₄, T₃, and heat stress survival time. Heritabilities were high for body weight, moderate for T₄ and heat stress survival time, and low for T₃. Phenotypic correlations were significant and negative for heat stress survival time with body weight and T₄, and for body weight with T₃ after TSH. Significant positive correlations were found for T₄ with T₃ after TSH and also T₄ and body weight. Analysis of genetic correlations suggested that none of the traits studied would be an adequate selection parameter for achieving heat tolerance without reducing body weight.Supported by State and Hatch funds allocated to the Georgia Agricultural Experiment Stations of the University of Georgia     

Computer simulation for hormones related to primary thyropathy

We propose a mathematical model of the human hypothalamus-anterior pituitary-thyroid system regulating basal metabolism, and practice computer simulation concerning primary thyropathy such as Graves' disease, hypothyroidism, T ₄-toxicosis and T ₃-toxicosis by use of this model. In order to throw light on properties of the system, indicial responses of the hormones, T ₄, T ₃, rT ₃, and TSH, and the function of the thyroid gland are computed. Medical treatments for Graves' disease and for hypothyroidism are simulated with a view to enhancing clinical significance. Performance of the simulation leads to an interesting result that when the convertion rate of blood T ₄ to blood T ₃ increases, explicit T ₃-toxicosis occurs, although the function of the thyroid gland is normal.