Congenital hypothyroidism, as studied in rats. Crucial role of maternal thyroxine but not of 3,5,3'-triiodothyronine in the protection of the fetal brain.

To study the protective effects of maternal thyroxine (T4) and 3,5,3'-triiodothyronine (T3) in congenital hypothyroidism, we gave pregnant rats methimazole (MMI), an antithyroid drug that crosses the placenta, and infused them with three different doses of T4 or T3. The concentrations of both T4 and T3 were determined in maternal and fetal plasma and tissues (obtained near term) by specific RIAs. Several thyroid hormone-dependent biological end-points were also measured. MMI treatment resulted in marked fetal T4 and T3 deficiency. Infusion of T4 into the mothers increased both these pools in a dose-dependent fashion. There was a preferential increase of T3 in the fetal brain. Thus, with a T4 dose maintaining maternal euthyroidism, fetal brain T3 reached normal values, although fetal plasma T4 was 40% of normal and plasma TSH was high. The infusion of T3 pool into the mothers increased the total fetal extrathyroidal T3 pool in a dose-dependent fashion. The fetal T4 pools were not increased, however, and this deprived the fetal brain (and possibly the pituitary) of local generation of T3 from T4. As a consequence, fetal brain T3 deficiency was not mitigated even when dams were infused with a toxic dose of T3. The results show that (a) there is a preferential protection of the brain of the hypothyroid fetus from T3 deficiency; (b) maternal T4, but not T3, plays a crucial role in this protection, and (c) any condition which lowers maternal T4 (including treatment with T3) is potentially harmful for the brain of a hypothyroid fetus. Recent confirmation of transplacental passage of T4 in women at term suggests that present results are relevant for human fetuses with impairment of thyroid function. Finding signs of hypothyroidism at birth does not necessarily mean that the brain was unprotected in utero, provided maternal T4 is normal. It is crucial to realize that maintainance of maternal "euthyroidism" is not sufficient, as despite hypothyroxinemia, the mothers may be clinically euthyroid if their T3 levels are normal.

Replacement therapy for hypothyroidism with thyroxine alone does not ensure euthyroidism in all tissues, as studied in thyroidectomized rats.

We have studied whether, or not, tissue-specific regulatory mechanisms provide normal 3,5,3'-triiodothyronine (T3) concentrations simultaneously in all tissues of a hypothyroid animal receiving thyroxine (T4), an assumption implicit in the replacement therapy of hypothyroid patients with T4 alone. Thyroidectomized rats were infused with placebo or 1 of 10 T4 doses (0.2-8.0 micrograms per 100 grams of body weight per day). Placebo-infused intact rats served as controls. Plasma and 10 tissues were obtained after 12-13 d of infusion. Plasma thyrotropin and plasma and tissue T4 and T3 were determined by RIA. Iodothyronine-deiodinase activities were assayed using cerebral cortex, liver, and lung. No single dose of T4 was able to restore normal plasma thyrotropin, T4 and T3, as well as T4 and T3 in all tissues, or at least to restore T3 simultaneously in plasma and all tissues. Moreover, in most tissues, the dose of T4 needed to ensure normal T3 levels resulted in supraphysiological T4 concentrations. Notable exceptions were the cortex, brown adipose tissue, and cerebellum, which maintained T3 homeostasis over a wide range of plasma T4 and T3 levels. Deiodinase activities explained some, but not all, of the tissue-specific and dose related changes in tissue T3 concentrations. In conclusion, euthyroidism is not restored in plasma and all tissues of thyroidectomized rats on T4 alone. These results may well be pertinent to patients on T4 replacement therapy.

Ontogenesis of thyroid function and interactions with maternal function.

Fetal and neonatal development of thyroid function involves the embryogenesis, differentiation and maturation of the thyroid gland, of the hypothalamic-pituitary-thyroid axis and of the systems controlling thyroid hormone metabolism. We focus here on aspects related to neurodevelopment. Throughout gestation, thyroxine (T4) transferred from the mother, present in embryonic fluids by 4 weeks, protects the fetal brain. Free T4 (FT4) in fetal fluids increases rapidly, approaching adult levels by midgestation, in concentrations that are determined by the maternal serum T4. T3 remains very low throughout pregnancy. In the cerebral cortex T3, generated from T4, reaches adult values by midgestation and is partly bound to specific nuclear receptor isoforms. The iodothyronine deiodinases are important for the spatial and temporal presence of T3 in different fetal brain areas. After onset of fetal thyroid secretion at midgestation, maternal transfer of T4 continues to contribute importantly to fetal serum T4, protecting neurodevelopment until birth. In rats, even a transient period of maternal hypothyroxinemia disrupts neurodevelopment irreversibly, supporting epidemiological evidence for its negative role in human neurodevelopment. The prompt treatment of maternal hypothyroidism or hypothyroxinemia should mitigate negative effects on neurodevelopment. Neurodevelopmental deficits of preterm infants might also result from an untimely interruption of the maternal transfer of T4 [Morreale de Escobar et al: J Clin Endocrinol Metab 2000;85:3975-3987; Best Pract Res Clin Endocrinol Metab 2004;18:225-248; Eur J Endocrinol 2004;151(suppl 3):U25-U37].

Conversion of L-thyroxine to triiodo-L-thyronine and biological activity of L-thyroxine, as measured by changes in growth hormone.

Pituitary growth hormone (GH) content and plasma GH response to pentobarbital (PB) of severely hypothyroid rats are markedly decreased compared with euthyroid rats. Both pituitary and plasma GH may be increased by single ip injections of physiologic doses of thyroxine (T4) (1.75 mug/100g BW) or triiodothyronine (T3) (0.2 mug/100g BW). Using this increase in GH of hypothyroid rats to measure the biological effectiveness of single doses of both iodothyronines, we have shown that the administration of 6-propyl-2-thiouracil (PTU) interferes with the activity of T4, but not with that of T3. It is known that deiodination of T4 and of T3 and conversion of T4 to T3 are partially inhibited by PTU. Therefore, it appears that deiodination of T4 is biologically important, whereas deiodination of T3 is not. Data presented here are thus consistent with the hypothesis that conversion of T4 to T3 in vivo plays an important role in the expression of T4 activity.

Thyrotropin-releasing hormone and thyroid hormone interactions on thyrotropin secretion in the rat: lack of inhibiting effects of small doses of triiodo-L-thyronine in the hypothyroid rat.

Rats were thyroidectomized (T) and injected once daily with thyroxine (T4) or triiodothyronine (T3) ip; circulating thyrotropin (TSH) levels and TSH response to 100 ng of thyrotropin releasing hormone (TRH) iv, were measured in different groups of rats at several intervals after the last dose of T4 or T3. It was found that T rats on 1.8 mug T4 or 0.4 mug T3/100 g BW/day, response to TRH decreased after the injection of the hormone, maximum suppressive effect being found about 7-8 h after T4, or 4 h after T3. The response increased as T4 or T3 levels reached a nadir, in agreement with present views on TRH, T4, and T3 interactions at the pituitary level. The degree of TSH response to TRH appears as a sensitive parameter of T4 or T3 activity in this experimental model. However, in T rats on 0.2 mug T3/100 g BW/day, TSH response to TRH did not decrease, but actually increased, after the daily injection of T3. These animals appeared to be in a state of continuous thyroid hormone deficiency. The same 0.2 mug T3 dose effectively suppresses the elevated basal TSH levels of these animals. It is also capable of decreasing TSH response to 100 ng TRH in animals under more "euthyroid" conditions. These results in the T rats on 0.2 mug T3 are not easily fitted into the relatively simple model frequently described to explain TRH-T3 interactions and TSH secretion.

Rapid effects of single small doses of L-thyroxine and triiodo-L-thyronine on growth hormone, as studied in the rat by radioimmunoassy.

The effects of thyroid hormone deprivation and restitution on growth hormone (GH) economy have been studied in the rat by means of a specific radioimmunoassay. The pituitary GH content and the plasma GH levels before and during stimulation with pentobarbital ("PB-test") were studied in male rats at different intervals after surgical thyroidectomy (T), and in T rats at different time intervals after the ip injection of 0.20, 1.75, and 5.0 mug thyroxine (T4) or 0.05, 0.10, 0.20 and 1.0 mug triiodothyronine (T3), all doses being referred to 100 g body wt. Pituitary GH content decreased very rapidly after T, a difference being shown at the end of the shortest time interval studied (24 h); 24 days after T, pituitary GH content was 0.3 percent or less of the pre-T level, the basal plasma GH was lower than in intact controls and an increase in plasma GH during PB-stimulation was no longer observed. When rats T for 30 days or longer were injected once with T4 or T3, pituitary GH content increased; basal plasma GH levels increased also and a positive response to PB was observed. An effect on pituitary GH content could be observed as soon as 2 h after the ip injection of 1.0 mug T3, or 6 h after 5.0 mug T4. The "latent period" was somewhat longer when lower doses of the hormones were used. Effects of a single 0.10 mug T3 dose could be detected within 12 h L-T3 appeared to be at least 9 times more potent in vivo tha T4, as assessed from the effect on pituitary GH. The mea-urement by RIA of changes in GH content of the rat pituitary may thus provide the most adequate parameter available at present (other than suppression of TRH-induced TSH release) for a biological effect in vivo of single doses of the thyroid hormones, a measurement clearly related to an important physiological role.

Concentrations of triiodo-L-thyronine in the plasma and tissues of normal rats, as determined by radioimmunoassay: comparison with results obtained by an isotopic equilibrium technique.

The concentrations of T3 (3',3,5-triiodo-L-thyronine) have been determined by RIA in plasma, liver, kidney, heart, muscle, brain, and lungs of normal male rats. The method involves homogenization, addition of a high specific activity labeled T3 tracer, extraction with ethanol, separation by paper chromatography, elution of the labeled T3 spot, evaporation of an aliquot, and assay of its T3 content by a highly sensitive RIA. In order to validate the results thus obtained, they were compared with the T3 concentrations determined for the same samples using an isotopic equilibrium technique. The data obtained by RIA were in close agreement with those derived from the isotopic equilibrium technique, whenever the latter could be applied accurately. The present RIA method permits the determination of T3 in tissues without imposing the well known limitations of the isotopic equilibrium technique. The concentration of T3 was higher in the kidney than in the liver. In kidney, liver, brain, and lung, it was higher than the concentration in plasma. The concentration of T3 in heart was the same as that in plasma. Similarities and differences with respect to data reported by others are discussed.

Transfer of thyroxine from the mother to the rat fetus near term: effects on brain 3,5,3'-triiodothyronine deficiency.

It has recently been shown that thyroid hormones are transferred from the mother to the developing rat embryo early in gestation, before the onset of fetal thyroid function. We have now studied whether there is transfer of T4 from the mother to the fetus late in gestation when the fetal thyroid is impaired. Normal and thyroidectomized females were mated, given a goitrogen [methimazole (MMI)], starting before the onset of fetal thyroid function and until term, alone or together with a constant infusion of T4 (1.8 micrograms/100 g BW.day). T4 and T3 were determined by RIA in several maternal samples and in tissues from 21-day-old fetuses. The administration of MMI blocked the fetal thyroid, as assessed from the decreased thyroid concentrations of T4 and T3. The concentrations of both iodothyronines also decreased in placenta, thyroid, plasma, brain, liver, lung, and carcass of fetuses from MMI-treated dams. Infusion of T4 into such MMI-treated mothers partly avoided this decrease, and T4 levels increased in all fetal tissues to 41-57% of those in normal fetuses. In contrast to this, T4 infusion affected the concentration of T3 to varying degrees in different tissues. The T3 concentration in plasma and lung increased very little when the MMI-treated mother was infused with T4, but in the brain T3 reached concentrations comparable to those in normal fetuses. Results not only show transfer of T4 from the mother to the fetus near term, at least when the fetal thyroid is impaired, but also suggest that it might mitigate, or avoid, the adverse effects of such failure on the developing brain.

Effects of maternal hypothyroidism on the weight and thyroid hormone content of rat embryonic tissues, before and after onset of fetal thyroid function.

Embryonic tissues were obtained from normal (C) and thyroidectomized (T) rats between 9 and 21 days of pregnancy. We determined the number and weight, as well as the T4 and T3 contents (RIA), of 9- to 12-day-old embryotrophoblasts, of 13- to 21-day-old embryos and placentas, and of liver, lung, and brain from 20- and 21-day-old fetuses. T4 and T3 were found in all samples obtained from C dams, both before and after onset of fetal thyroid function. Despite low levels of both iodothyronines in fetal plasma near term, their concentrations in fetal brain and lung had reached half the maternal values. The T3/T4 ratio in fetal organs was the same, or higher, than in adult rats. Maternal thyroidectomy resulted in a marked decrease of the number and individual weights of viable conceptuses, throughout gestation. Fetal organ weights near term were also decreased, and changes were found in brain DNA and protein concentrations. T4 and T3 were undetectable in all embryotrophoblasts, embryos and placentas obtained from T dams before onset of fetal thyroid secretion. They were still markedly reduced in 21-day-old placentas. Total extrathyroidal contents of T3 and T4 in 20- and 21-day-old fetuses from T dams were also low as compared to those from normal mothers, but individual organs were not affected to the same degree. Thus concentrations were decreased in the carcass (whole embryo minus the trachea + thyroid + liver + lung + brain), but normal in the brain. These results show that maternal hypothyroidism is accompanied by thyroid hormone deficiency of the conceptus before the fetal thyroid functions. After this, alterations of T4 and T3 concentrations persist until term. Development is also delayed. Thus, adverse effects of maternal hypothyroidism may be due, at least in part, to the thyroid hormone deficiency of the embryonic tissues, and not only to the hypothyroid condition of the mother.

Contribution of maternal thyroxine to fetal thyroxine pools in normal rats near term.

Normal dams were equilibrated isotopically with [125I]T4 infused from 11 to 21 days of gestation, at which time maternal and fetal extrathyroidal tissues were obtained to determine their [125I]T4 and T4 contents. The specific activity of the [125I]T4 in the fetal tissues was lower than in maternal T4 pools. The extent of this change allows evaluation of the net contribution of maternal T4 to the fetal extrathyroidal T4 pools. At 21 days of gestation, near term, this represents 17.5 +/- 0.9% of the T4 in fetal tissues, a value considerably higher than previously calculated. The methodological approach was validated in dams given a goitrogen to block fetal thyroid function. The specific activities of the [125I]T4 in maternal and fetal T4 pools were then similar, confirming that in cases of fetal thyroid impairment the T4 in fetal tissues is determined by the maternal contribution. Thus, previous statements that in normal conditions fetal thyroid economy near term is totally independent of maternal thyroid status ought to be reconsidered.

Effects of maternal iodine deficiency on thyroid hormone economy of lactating dams and pups: maintenance of normal cerebral 3,5,3'-triiodo-L-thyronine concentrations in pups during major phases of brain development.

Female rats were fed a diet with a low iodine content (LID), or the same LID supplemented with KI, and mated. Fetuses were obtained at 17 and 21 days of gestation, or pups were killed at different ages after birth. The dams on LID were markedly iodine deficient and developed a large goiter. Their thyroidal iodine content was only 4% of that of LID + I dams. The iodine deficiency of the LID mothers was severe enough to result in very low plasma T4 levels and in hepatic and cerebral T3 deficiency, despite normal circulating levels of T3. The fetuses from LID dams had low concentrations of iodine in their placentas and thyroid glands, and were deficient both in T4 and T3 in all tissues studied, including the brain. After birth, however, suckling LID pups were able to increase the plasma T4 to levels which were higher than those found in either LID fetuses or in adult LID progeny, although plasma T4 was always lower than in age-paired LID + I animals. This increase in T4 was probably due to an approximately 5-fold increase in iodine intake while suckling. Milk from LID mothers was found to contain 22% of the amount of iodine found in milk from LID + I dams, in contrast to their iodine intake, which was about 4% that of the LID + I rats. Cerebral T3 levels were the same for LID and for LID + I pups throughout most of the postnatal period of brain development. This finding might explain the difficulties encountered in obtaining an experimental model of neurological cretinism in rats.

Effects of maternal iodine deficiency on the L-thyroxine and 3,5,3'-triiodo-L-thyronine contents of rat embryonic tissues before and after onset of fetal thyroid function.

Female rats were placed on a low iodine diet (LID) or LID supplemented with KI. They were mated 3-6 months later. Maternal and embryonic tissues were obtained both before the onset of fetal thyroid function, at 11 and 17 days of pregnancy, and at 21 days of gestation. T4 and T3 concentrations were measured by RIA. T4 concentrations were very low in the plasma, liver, and lung of LID dams and in all embryonic samples obtained from such mothers, namely 11-day-old embryotrophoblasts, 17-day-old placentas and embryos, 21-day-old placentas, embryos, plasma, liver, lung, and carcass (whole embryos minus the trachea, thyroid, blood, liver, and brain). T3 was low in 17-day-old placentas and embryos and in all fetal tissues obtained at 21 days of gestation from LID dams. These results show that when iodine deficiency is severe enough to result in very low maternal plasma T4 levels, embryonic tissues are deficient in T4 and T3 both before and after the onset of fetal thyroid function. This finding might be relevant to the etiopathology of human iodine deficiency disorders.

The rat placenta and the transfer of thyroid hormones from the mother to the fetus. Effects of maternal thyroid status.

We have studied the effects of maternal thyroid status on the effectiveness of the rat placenta near term as a barrier for the transfer of T4 and T3 to the fetus. Dams were given methimazole to minimize the fetal contribution to the T4 and T3 pools, so that the iodothyronines found in the conceptus are ultimately of maternal origin. The dams were infused with saline, or with T4 or T3 at doses ranging from 2.3-27.8 nmol T4 and from 0.77-20.7 nmol T3/100 g BW per day. A group of normal pregnant dams (C) was included. At 21 days of gestation T4, T3, and rT3 were measured by RIA in maternal and fetal plasma, and in maternal and fetal sides of the placenta. The total fetal extrathyroidal T4 and T3 pools were also determined. The dose-related changes in T4, T3, and rT3 levels in the placenta confirm the presence of both inner and outer ring iodothyronine deiodinase activities, and suggest increasing accumulation of the iodothyronines. Despite this, fetal extrathyroidal T4 and T3 increase progressively in T4-infused groups as a function of maternal circulating T4 levels. Fetal extrathyroidal T3 increases progressively in T3-infused groups as a function of maternal plasma T3. There was no evidence that the net maternal contribution of T4 or T3 would be proportionally less when the maternal pools became very high. It was concluded that the rat placenta is only a limited barrier for the transfer of T4 and T3 to the fetus.

Maternal nonthyroidal illness and fetal thyroid hormone status, as studied in the streptozotocin-induced diabetes mellitus rat model.

We have used the streptozotocin-induced diabetes mellitus pregnant rat as a model of maternal nonthyroidal illness. We measured the effects of different degrees of diabetes mellitus on maternal body weight, the outcome of pregnancy, circulating glucose, insulin, T4, T3, rT3, and TSH in mother and fetus, T4 and T3 in maternal and fetal tissues, and iodothyronine deiodinases in liver, lung, and brain. All of the changes in thyroid hormone status typical of nonthyroidal illnesses were observed in the mothers and were related to the degree of the metabolic imbalances. Most were controlled with a daily insulin dose of 0.5 U/100 g BW. Normalization of maternal placental T4, however, required higher insulin doses than in other maternal tissues. The number and body weight of the fetuses, their pituitary GH contents, and their thyroid hormone status were severely affected. The total extrathyroidal T4 and T3 pools decreased to one third of normal fetal values. T4 and T3 concentrations in the fetal brain were lower than normal, and the expected increase in type II 5'deiodinase activity was not observed. The low cerebral T3 only improved with adequate insulin treatment of the dams. It is concluded that maternal diabetes mellitus, and possibly other nonthyroidal illnesses that impair the availability of intracellular energy stores, may affect fetal brain T3 when thyroid hormones are essential for normal development.

Thyroid hormones influence serum leptin concentrations in the rat.

Leptin, the product of the ob gene, is secreted by adipocytes and has been shown to decrease appetite and increase energy expenditure. Leptin mRNA in adipocytes correlates with body wt, and serum leptin levels correlate with body fat. Alterations in thyroid status are frequently associated with changes in body wt. To evaluate the possible influence of thyroid status on the leptin system, we have measured serum leptin concentrations in thyroidectomized rats infused either with placebo, or with different doses of T4 (0.8 to 8.0 microg/100 g body wt per day) or T3 (0.25 to 2.0 microg/100 g body wt per day), covering a wide range of thyroid hormone concentrations, from overt hypothyroidism to hyperthyroidism. Intact animals infused with placebo were used as euthyroid controls. Infusion of T4 or T3 into thyroidectomized rats resulted in a decrease in serum leptin levels with respect to the thyroidectomized animals infused with placebo. When compared to the control group, serum leptin levels were decreased in the groups infused with the higher T4 and T3 doses, and tended to be elevated in the thyroidectomized animals infused with placebo. The leptin/body wt ratio was markedly increased in thyroidectomized rats infused with placebo, and decreased in the animals infused with the higher thyroid hormone doses. In conclusion, thyroid hormones exert a negative influence on serum leptin concentrations, which is greater than expected by the changes in body wt The precise mechanism of this influence remains to be elucidated.

Regulation of uncoupling protein messenger ribonucleic acid and 5'-deiodinase activity by thyroid hormones in fetal brown adipose tissue.

We studied the regulation of type II 5'-deiodinase (5'D-II) activity and uncoupling protein (UCP) messenger RNA (mRNA) by thyroid hormones in fetal brown adipose tissue (BAT). Fetuses were obtained from hypothyroid pregnant rats infused with increasing doses of T4 or T3. Infusion of T4 into hypothyroid pregnant rats increased T4 and T3 concentrations and inhibited 5'D-II activity in fetal BAT. In contrast, infusion of T3 increased BAT 5'D-II activities at low, normal, or high BAT T3 concentrations. The relationship between thyroid hormone concentrations in fetal BAT and plasma showed that BAT T3 concentrations are relatively stable, increasing less than 2-fold over a wide range of circulating T4 (3-fold) or T3 (8-fold) concentrations. Most T3 in fetal BAT are locally derived from T4 and not from plasma T3. UCP mRNA expression decreased to 30% of control values in hypothyroid fetuses. UCP mRNA levels were restored to normal in parallel with BAT T3 concentrations after the infusion of either T4 or T3. UCP mRNA levels correlate well with BAT T3 concentrations. Supraphysiological doses of T4 did not further increase either BAT T3 or UCP mRNA levels. T3 might regulate basal UCP mRNA expression during fetal life.

Thyroid hormones in tissues from fetal and adult rats.

Concentrations of T4 and T3 were recently measured in rat fetal tissues, and the reported values were found to be more than 10-fold higher than those found by us. The differences have been explained by the assumption that previous analytical procedures, neither avoid deiodination during autopsy of the animals or during extraction and purification, because phloretin [(3'),4',4,6-(tetra)trihydroxyaurone], a potent inhibitor of 5'-iodothyronine deiodinase activity in vitro, had not been used to prevent such problems. We here show that perfusion with phloretin during autopsy does not affect 5'-iodothyronine activity or T4 and T3 concentrations in liver, kidney, or brain. Evidence is also provided that the addition of phloretin during the homogenization process is superfluous, as the use of 80% ethanol and 0.02 M NaOH for this step results in undetectable deiodinase activity. Data are presented showing that during the final sample drying, no losses or degradation of T4 and T3 occur, confirming the adequacy of the individual recovery corrections using radiolabeled iodothyronines as internal tracers. We also present quantitative information on the intralaboratory variability of the T4 and T3 concentrations found in tissues from normal fetuses and their mothers as well as in adult males and nonpregnant females. Results are comparable to those obtained by others using entirely different analytical procedures.

Adult-onset hypothyroidism and the cerebral metabolism of (1,2-13C2) acetate as detected by 13C nuclear magnetic resonance.

The effects of adult-onset hypothyroidism on the metabolic compartmentation of the cerebral tricarboxylic acid cycle and the gamma-aminobutyric acid (GABA) shunt have been investigated by 13C nuclear magnetic resonance spectroscopy. Rats thyroidectomized as adults and age-matched controls were infused in the right jugular vein with unlabeled or (1,2-13C2) acetate solutions for 60 min. At the end of the infusion, the brains were frozen in situ and perchloric acid extracts were prepared and analyzed by 13C nuclear magnetic resonance and reverse-phase HPLC. Thyroidectomized animals showed a decrease in the incorporation of 13C from (1,2-13C2) acetate in cerebral metabolites and an increase in the concentrations of unlabeled glutamate and GABA. Computer-assisted interpretation of the 13C multiplets observed for the carbons of glutamate, glutamine, and GABA indicated that adult-onset hypothyroidism produced 1) a decrease in the contribution of infused (1,2-13C2) acetate to the glial tricarboxylic acid cycle; 2) an increase in the contribution of unlabeled acetyl-CoA to the neuronal tricarboxylic acid cycle; and 3) impairments in the exchange of glutamate, glutamine, and GABA between the neuronal and glial compartments. Despite the fact that the adult brain has often been considered metabolically unresponsive to thyroid hormone status, present results show metabolic alterations in the neuronal and glial compartments that are reversible with substitution therapy.

Evidence against a major role of L-thyroxine at the pituitary level: studies in rats treated with iopanoic acid (telepaque).

To determine whether T4 has an intrinsic effect at the pituitary level, it would be important to block conversion of T4 to T3 completely. We have attempted to achieve this with iopanoic acid (IOP), a radiographic contrast agent. We have then measured in the same animals the effects of such treatment on the conversion of T4 to T3 or on the deiodination of T3 and on the pituitary response to a dose of T4 or T3. Plasma TSH levels and pituitary GH content were measured as biological end points. Thyroidectomized rats were injected with a single dose of T4 (1.7 micrograms/100 g BW) labeled with [125I]T4 (Exp A) or with a single dose of T3 (0.33 microgram/100 g BW) labeled with [125I]T3 (Exp B) and treated with IOP or solvent. Animals of Exp A were killed 24 h after iodothyronine injection and those of Exp B were killed 4, 12, and 24 h after injection of the iodothyronine. The concentrations of [125I]T4 and [125I]T3 were measured in several tissues, including the anterior pituitary, after extraction and paper chromatography and quantified with the aid of 131I-labeled markers added in vitro. Plasma and pituitary T3 and T4 plasma TSH, and pituitary GH were measured by specific RIAs. Results show that treatment with IOP markedly inhibits the conversion of T4 to T3 and the deiodination of T3. In IOP-treated thyroidectomized rats, the injection of T4 results in little, if any, effect at the pituitary level, despite an almost 3-fold increase in the percentage of injected T4 found in the gland. Treatment with IOP does not inhibit the effects of a T3 dose; if anything, they appear to be enhanced. It is concluded that, as assessed from biological responses involving the anterior pituitary, a dose of T4 has little if any effect other than that which can be attributed to the T3 generated from it.

Presence of L-thyroxine and 3,5,3'-triiodo-L-thyronine in tissues from thyroidectomized rats.

Female rats were killed 15 days, 2 months, and 4 months after surgical thyroidectomy that was followed by injection of 100 microCi 131I. The concentrations of T3 and T4 were measured in tissues (liver, kidney, brain, heart, and hindleg muscle) specific RIAs. Results were compared to those found in intact rats. Thyroidectomy resulted in severe hypothyroidism by 2 and 4 months after the operation, as assessed by undetectable levels of T4 and T3 in unextracted plasma, high circulating TSH, hypothermia, stasis of body weight increase, and depletion of pituitary GH content. Concentrations of T4 and T3 in plasma, as determined after extraction and concentration, were very low, being less than 5% of the normal value by the earliest observation period (15 days). In contrast, although tissue concentrations and total organ contents also decreased after thyroidectomy, they were still clearly detectable 4 months after thyroidectomy. The rates of decrease of T4 and T3 concentrations in most tissues were markedly slower than expected from their rapid decrease in plasma. Some tissues still contained 20% of the normal level 2-4 months after ablation of the thyroid. Tissue levels of thyroid hormones were hardly detectable in rats thyroidectomized 6 months before, having decreased in most tissues to less than 5% of the normal value. Several animals from this group had died. It is concluded that tissues from severely hypothyroid thyroidectomized rats may contain higher concentrations of T4 and T3 than previously thought. The idea that thyroid hormone is not essential for life, based on the assumption that thyroidectomized animals survive without thyroid hormones, might have to be reevaluated.