Altered thyroidal responsivity to thyrotropin induced by circulating thyroid hormones. A "short-loop" regulatory mechanism?

The targeted inactivation of TRβ gene in thyroid follicular cells suggests a new mechanism of regulation of thyroid hormone production

Thyroid epithelial cells, or thyrocytes, express functional thyroid hormone receptors but no precise role has yet been assigned to either TRα or TRβ in the thyroid gland. In this study, we analyzed the impact of inactivating the TRβ gene in the thyroid of mice. First, we generated a mouse line named Thyr-Cre, expressing the Cre recombinase under the control of the thyroglobulin gene promoter, which led to a complete recombination of floxed genes in thyrocytes. Thyr-Cre mice were then crossed with TRβ floxed mice (TRβ(flox/flox)) to obtain a thyrocyte-selective deletion of TRβ. Thyr-TRβ(-/-) mice were characterized by a decrease in the size and functional activity of the thyroid gland. These alterations were associated with a decrease in plasma TSH concentration. Surprisingly, Thyr-TRβ(-/-) displayed elevated serum T(4) and rT(3) concentrations with no significant change in serum T(3) levels. Their intrathyroidal free T(4) and rT(3) contents were also elevated, whereas the ratio of serum T(4) to thyroid free T(4) was decreased by comparison with wild-type littermates. Also, within the thyroid, deiodinases D1 and D2 were reduced as well as the expression levels of genes encoding monocarboxylate transporters (Mct8 and Mct10). Such a decrease in intrathyroidal deiodination of T(4) and in the expression of genes encoding thyroid hormone transporters may contribute to the primary overproduction of T(4) observed in Thyr-TRβ(-/-) mice. In conclusion, these data show that the control of thyroid hormone production involves not only TRβ-dependent mechanisms acting at the level of hypothalamus and pituitary but also TRβ-dependent mechanisms acting at the thyroid level.

TSH and Thyrotropic Agonists: Key Actors in Thyroid Homeostasis

This paper provides the reader with an overview of our current knowledge of hypothalamic-pituitary-thyroid feedback from a cybernetic standpoint. Over the past decades we have gained a plethora of information from biochemical, clinical, and epidemiological investigation, especially on the role of TSH and other thyrotropic agonists as critical components of this complex relationship. Integrating these data into a systems perspective delivers new insights into static and dynamic behaviour of thyroid homeostasis. Explicit usage of this information with mathematical methods promises to deliver a better understanding of thyrotropic feedback control and new options for personalised diagnosis of thyroid dysfunction and targeted therapy, also by permitting a new perspective on the conundrum of the TSH reference range.

Analysis of the functional state of T3 nuclear receptors expressed in thyroid cells

T3 nuclear receptors (TR) are present in thyroid cells. We have analyzed the ability of thyroid TR to function as transcriptional regulators. Studies were performed on pig thyrocytes in primary culture. Messenger RNA corresponding to TR alpha 1, alpha 2 and beta were detected in pig thyrocytes by RT-PCR and Northern blot; the alpha 2 mRNA was more abundant than the alpha 1 mRNA. Thyrocytes were transiently transfected with different plasmids containing the CAT (chloramphenicol acetyl transferase) gene placed under the control of different promoters (delta MTV, TK or delta SV40) and bearing a thyroid hormone response element, TREp or TRE DR + 4. It was found that TSH induced a concentration-dependent increase of the transfection efficiency, an effect reproduced by (Bu)2cAMP and Forskolin. Cells transfected with either delta MTV-, TK- or delta SV40-TREp-CAT expressed similar basal CAT activities. Addition of T3 produced a 3-fold increase of CAT activity expressed from each of these vectors. In contrast, CAT activity expressed from a vector containing the TRE DR + 4 was decreased by about 50% by T3. Thus, TREp and TRE DR + 4 gave distinct responses. These data demonstrate that TR physiologically expressed in thyroid cells can act as transcriptional regulators in a T3-dependent manner. This finding directly substantiates the concept of autocrine regulatory actions of thyroid hormones.

Inhibition of human thyroid adenylyl cyclase by 2-iodoaldehydes

2-Iodohexadecanal (IHDA), which can be formed upon addition of iodine to the vinyl ether group of plasmalogens, has been identified as a major thyroid iodolipid (Pereira et al. (1990) J. Biol. Chem. 265, 17018-17025). In this study, we have investigated the possibility that it would be a mediator of the inhibitory effect of iodide on thyroid adenylyl cyclase. In human thyroid membranes, IHDA inhibited the adenylyl cyclase activity stimulated by thyrotropin (TSH), GTP-gamma-S or forskolin (FSK), whereas it did not decrease the specific binding of TSH to its receptors. The inhibitory effect on the cyclase reached a maximum after a 1-h-pre-incubation of the membranes with IHDA at 30 degrees C and was poorly reversible. It was also observed following a 4-h incubation with IHDA at 4 degrees C, a condition in which adenylyl cyclase is protected against heat inactivation. IHDA decreased the Vmax of adenylyl cyclase, but had no effect on the Km for ATPMg2-.IHDA also inhibited the FSK-stimulated adenylyl cyclase activity in liver and kidney cortex membranes, but had no effect on the Mg(2+)-ATPase activity of thyroid membranes. The inhibitory effect of IHDA has also been demonstrated in intact cells. As in membranes, IHDA decreased the rise in cAMP induced by TSH in cultured dog thyroid cells and this inhibition was maintained following pretreatment of the cells with pertussis toxin. In order to evaluate the specificity of the IHDA action, various analogs have been synthesized. This study has permitted the identification of two major structural features required for the inhibition of human thyroid adenylyl cyclase; the terminal aldehyde function and an iodine atom at C2, other halogens being ineffective. In conclusion, we have shown that IHDA exerts a direct inhibitory effect at or near adenylyl cyclase; all the properties of this effect characterized so far are identical to those of the adenylyl cyclase inhibition obtained following the exposure of thyroid tissue to iodide.

Differential regulation of multiple c-erbA expression by thyrotropin, insulin and insulin-like growth factor I in rat thyroid FRTL-5 cells

Regulation of multiple c-erbA gene expression was studied in rat thyroid FRTL-5 cells. Two species of erbA alpha (alpha 1 and alpha 2) mRNA and one species of erbA beta (beta 1) mRNA were identified by Northern blot analysis. Withdrawal of thyrotropin (TSH), insulin and serum from the complete medium resulted in an increase in alpha 1 and alpha 2 erbA mRNA levels without altering the level of erbA beta 1 mRNA. Readdition of TSH, N6,2'-O-dibutyryl cAMP or forskolin caused a transient reduction of alpha 1 and alpha 2 mRNA levels (75-90%) at 3-12 h. The alpha 1 and alpha 2 mRNA levels were restored at 24 h. The TSH action was dose-dependent showing the half-maximal effect at around 10(-9) M. Readdition of TSH did not show any effect on beta 1 mRNA level. The action of TSH was not dependent on ongoing protein synthesis but required ongoing transcription. Inhibitors of thyroid hormone biosynthesis, propylthiouracil and methylmercaptoimidazole, did not show any effect on TSH action. Readdition of insulin or insulin-like growth factor I (IGF-I) caused a dose-dependent reduction of alpha 1 and alpha 2 mRNA levels without any effect on beta 1 mRNA level. Their action was slower than TSH and persistent. The actions of insulin and IGF-I were dependent on both ongoing translation and transcription. These results indicate that TSH and insulin/IGF-I reduce levels of c-erbA alpha 1 and alpha 2 mRNA possibly by two distinct mechanisms without altering c-erbA beta 1 mRNA level in FRTL-5 cells.

Thyroid hormone inhibition of intermediary metabolism in dog thyroid slices stimulated by different agonists

The role of thyroid hormones in a short loop feedback in the thyroid is controversial. This process was studied in dog thyroid slices stimulated by TSH, carbachol and phorbol esters. Incubation of thyroid slices with T3 and T4 for 1 hour inhibited the subsequent stimulation of glucose oxidation induced by carbachol and phorbol esters but not by TSH. T3 also inhibited the stimulation of 32P incorporation into phospholipids stimulated by these two agonists. Glucose oxidation stimulated by TSH, carbachol and 12-0-tetradecanoyl-phorbol-13-acetate (TPA) was inhibited by rT3 and the inhibition was not reversed by methimazole, which did abolish the inhibition induced by iodide, MIT and DIT. TSH stimulation of cAMP was not blocked by T3 or T4 but was by rT3 and MIT- and DIT. The mechanism of such inhibition appears to be complex, possibly involving formation of iodide from rT3, MIT and DIT but also dependent on the intact iodothyronine. Moreover, our data suggest that T3 and T4 exert their inhibition on the thyroid through the phospholipids cascade and this mechanism is probably independent on the release of iodide from these iodocompounds.

Further characterization of the iodide inhibitory effect on the cyclic AMP system in dog thyroid slices

Iodide inhibits cyclic AMP accumulation in the thyroid by a process which is prevented by inhibition of iodide uptake and of thyroid peroxidase. By a similar process, it also exerts other independent effects such as the enhancement of iodinated protein release. Iodide inhibited the stimulation of adenylate cyclase by prostaglandin E1, cholera toxin and forskolin. The action of iodide was not relieved by phosphodiesterase inhibitors and was not additive with the effect of norepinephrine or adenosine. Iodide did not decrease the cellular level of ATP. The data are compatible with an inhibition of adenylate cyclase beyond the level of the receptor, presumably at the level of the catalytic unit or its interaction with the positive transducing unit NS. The effect of iodide required TSH for its expression but not for its installation. It was decreased under all conditions in which iodide organification was decreased: decreased iodide or increased methimazole concentration, absence of calcium in the medium, etc. However, the relation between iodide binding to proteins and effect was not linear. The effect was not relieved by washing in the absence of iodide and in the presence of perchlorate, but it was partly reversible in the presence of methimazole propylthiouracyl or thiourea. It was not relieved by cooling to 20 degrees C and cytochalasin b, which block stimulated thyroglobulin hydrolysis and iodothyronine release, nor by actinomycin D, cycloheximide, puromycin, mepacrine or indomethacin. The data suggest that iodide binds to a saturable cell component by a reaction which is reversible only in the presence of thiol-containing drugs.

A stable, reproducible radioreceptorassay for thyrotropin binding inhibiting immunoglobulins (TBII)

A radioreceptorassay for thyrotropin binding inhibiting immunoglobulins (TBII) is presented. A stable reproducible method was established based on the use of thyroid glands obtained by autopsy for the preparation of a highly purified plasma membrane from human thyroid homogenate. Lower detection limit for TSH was below 5 miu/l. The TBII activity was measured in a crude IgG precipitate from serum and the results were calculated as an index value compared to normal controls, the mean of which was defined as 1.00. The binding of [125I] TSH to this membrane preparation was stable for more than 6 months. Interassay variations of the TBII index in three ranges were 1.30 +/- 0.19, 0.37 +/- 0.08 and 0.04 +/- 0.07 (mean +/- SD) over a period of 6 months. The intra-assay variations of the TBII index in three ranges were 1.30 +/- 0.08, 0.46 +/- 0.03 and 0.09 +/- 0.03 (mean +/- SD). Seventy-six of 116 patients with untreated Graves' disease had index values below the reference interval and TBII was also detected in 4 of 14 patients with multinodular goitre. The value of the TBII index in patients with untreated Graves' disease was significantly correlated to S-T3, the 4 h [131I] uptake and the 20 min [99mTc] uptake. No significant correlation between the TBII index and long-acting thyroid stimulator (LATS) measured in a bioassay was found. However, the LATS-positive patients also had positive TBII index.

Uptake, metabolism and action of triiodothyronine in calf-thyroid slices

T3 and T4 at 10(-6)M caused a significant inhibition on [3H]uridine incorporation into RNA in calf-thyroid slices. This effect was not altered by addition of 10(-3) M PTU or MMI. The metabolism of 125I-T3 was studied under the same conditions. There was a very slight dehalogenation after 60 min (less than 5%) which was inhibited by PTU. The time course of the uptake of labeled T3 showed a temperature dependence, and this uptake was not altered by 10(-6)M KI, 1-T4, DIT or MIT, thus indicating specificity. The early phase of labeled T3 entrance into the cell was inhibited by the addition of colt T3 in a dose- dependent manner from 10(-9) to 10(-5)M. Slices previously stimulated by cAMP or CGMP showed a significant increase in the uptake of labeled T3. ATPase activity or ATP, protein or RNA synthesis does not seem to play a role in this process. The relationship between substitutions in the thyronine molecule and its biological action on RNA synthesis was also studied, and some preliminary conclusions were drawn. These studies demonstrate that T3 has a direct action on the thyroid.

Interaction between iodothyronines and thyrotropin receptor in human cultured thyroid cells

In order to verify the existence of a "short-loop" negative feedback between iodothyronines and adenylate cyclase system of human thyroid, we have studied the effect of preincubation with iodothyronines, iodotyrosines, iodothyronine analogues and iodide on TSH-induced cAMP cellular accumulation in normal human thyroid cells in primary culture. Iodide did not produce an inhibitory effect on TSH-dependent adenylate cyclase system both in normal human thyroid plasma membranes and cultured cells. Iodothyronines at a 30-40 microM concentration did not inhibit the TSH-dependent adenylate cyclase activity of human thyroid plasma membranes; however at a 1 microM concentration they were able to inhibit the TSH-dependent cAMP accumulation by cultured cells. Preincubation with iodotyrosines and iodothyronine analogues failed to inhibit the TSH-responsive cAMP accumulation in human thyroid cultured cells.

Thyroid-hormone effects on ornithine decarboxylase

We examined thyroidectomized, normal and hyperthyroid rats and found that ornithine decarboxylase activity was directly correlated with thyroid functional state in heart and liver and unaffected in brain, testes and spleen, phenomena that correlate with the known effect of thyroid hormone on protein synthesis.

Biochemistry of thyroid regulation under normal and abnormal conditions

Perhaps in an oversimplified view, abnormal thyroid growth can be classified into two main categories: a) those cases due to excess of thyroid stimulators extrinsic to the gland; b) situations in which an intrinsic alteration in the gland occurs: Extrinsic (excess thyroid stimulation) Iodide deficiency with elevated TSH Goitrogens Graves' immunoglobulins Thyroid stimulating factors produced by tumors Dishormonogenesis with hypothyroidism Intrinsic (normal TSH) Increased sensitivity to TSH (iodine depletion) Altered autoregulation (?) Abnormal TSH receptor Other biochemical abnormalities From the studies performed in animals it can be concluded that since goiter appears before a detectable increase in serum TSH occurs, an intrinsic alteration in the thyroid gland would be responsible for the onset of growth. Under these conditions TSH would play a permissive role in promoting and maintaining the gland enlargement. In some aspects this situation is similar to that of certain endemic goiter areas. It may be postulated that under a mild iodine deficiency a decrease in thyroidal iodine concentration occurs (and/or in certain iodocompounds), thus rendering the gland more sensitive to the stimulatory action of TSH, and leading to the appearance of goiter. If this mechanism is able to maintain an euthyroid status no further alterations will occur. In more severely iodine deficient areas, or when additional factors such as dietary goitrogens are present, hypothyroidism develops and TSH is clearly elevated. A similar localized mechanism can be postulated for the development of nodular goiter. It is more difficult to explain the pathogenesis of goiter and tumors in nonendemic areas, since the biochemical findings so far reported are not conclusive. It seems likely that an alteration of the TSH receptor is a common factor to many tumors in man and animals. However, some contradictory results would preclude us from making a general statement. The wide variety of biochemical alterations reported would perhaps indicate, that there is not a single cause for the rise of abnormal thyroid growth and that different factors may play a role in the regulation of growth under such circumstances. It is to be hoped that future studies will provide a better comprehension of this problem.

In vitro and in vivo refractoriness to thyrotropin stimulation of iodine organification and thyroid hormone secretion

Earlier studies indicated that initial exposure of thyroid slices to thyrotropin diminished responsiveness of the adenylate cyclase-cyclic AMP system, glucose oxidation, and (32)P(i) incorporation into phospholipids upon readdition of the hormone. The present studies demonstrate that slices from dog, beef, and human thyroid glands initially incubated with thyrotropin (TSH) were less responsive to subsequent addition of the hormone when organification of iodide was examined. Increasing the amount of TSH did not overcome the refractoriness induced by the initial exposure to the hormone. Furthermore, the stimulatory effects of dibutyryl cyclic AMP and prostagladin E(1) were abolished in slices previously incubated with TSH. Development of such refractoriness did not depend upon new protein synthesis and was not abolished by 1 mM prophylthiouracil in the first incubation. Addition of 0.1 muM thyroxine or triiodothyronine or 1.5 muM iodide during all three incubations did not modify the response to TSH, added for the first time in the third incubation. However, 1 mM iodide in the buffer during all three incubations inhibited the response to TSH during the third incubation. During the refractory period, effects of TSH on colloid droplet formation were also diminished. The in vivo effect of TSH on serum l- triiodothyronine in rats was significantly reduced when the rats had been injected with TSH 8 h earlier. These studies demonstrate that TSH-induced refractoriness also includes effects on organification of iodine and secretion of thyroid hormone. The results cannot be adequately explained by unresponsiveness of adenylate cyclase because effects of dibutyryl cyclic AMP and prostagladin E(1) were also inhibited by prior exposure to TSH.

Hormonal regulation of thyroidal protein phosphokinase activities. II. Differential sensitivity of type I and type II cAMP-dependent enzymes to the treatment of rats with thyroxine

Two cAMP-dependent protein kinase activities can be separated from rat thyroid cytosol. Their elution properties during the gel chromatography, as well as their sedimentation coefficients after sucrose-gradient ultracentrifugation and their capacity to dissociate in the presence of histones, suggest they are like the Type I and Type II protein kinases described in many other tissues. The sensitivity of the two types of kinases to thyroxine treatment is different. The activity of Type I is not changed during the first 5 days of treatment. Thereafter, it decreased by about 50% and is maintained at that level for up to one month. The activity of the Type II enzyme decreased rapidly by about 30-40% already on the second day of treatment, and after 10 days it reached 50% of the initial level. This differential reactivity of the two types of enzymes to the thyroxine treatment leads to profound modifications in their relative activities between the second and the fifth day of treatment. The significance of these results and the possible role of the two types of kinases in the control of different steps of iodine metabolism have been discussed.

Stimulation by triiodothyronine of the in vitro uptake of sugars by rat thymocytes

Studies were conducted to ascertain the in vitro effect of 3,5,3'-triiodothyronine (T(3)) on the accumulation of the glucose analogue, 2-deoxyglucose (2-DG), by thymocytes freshly isolated from weanling rats. At a concentration of 1 muM, T(3) stimulated the 15-min uptake of (3)H-2-DG after cells had been exposed to T(3) for only 30 min. Significant stimulation of 2-DG accumulation was produced by 1 nM T(3), with increasing stimulation at doses ranging up to 10 muM. T(3) did not alter the fraction of accumulated 2-DG that was phosphorylated, and kinetic studies indicated that its effect was associated with a significant increase in the apparent V(max) of 2-DG accumulation, but not the apparent K(m). T(3) also enhanced the accumulation by thymocytes of the nonmetabolized glucose analogue, 3-O-methylglucose (3-O-MG), an effect that was evidently the result of an increase in 3-O-MG transport into the cell, because it was seen in cells incubated with (3)H-3-O-MG for only 30 s. The proportionate increase in 2-DG accumulation produced by T(3) was not altered by preincubating cells with concentrations of puromycin or cycloheximide sufficient to reduce [(3)H]-leucine incorporation by 95%, and T(3) over a period of >2 h had no effect on [(3)H]leucine incorporation itself. These results indicate that T(3) stimulates the uptake of sugars in rat thymocytes in vitro by an effect on their inward transport. The promptness of the effect and its failure to be inhibited during profound inhibition of protein synthesis further indicate that this effect of T(3) is not mediated through a nuclear-dependent mechanism. Rather, the properties of this response, and of the increases in amino acid and 2-DG accumulation produced by T(3) in other tissue preparations, strongly suggest that these effects of T(3) are mediated at the level of cell membrane.

Inhibition of thyroidal cyclic AMP-dependent protein kinase by thyroid hormone

Bovine thyroid cyclic AMP-dependent protein kinase was purified by DEAE-Sephadex and Sephadex G-200 chromatography. This preparation showed a 240-fold increase in specific activity over the initial 20,000 x g supernatant with histone as substrate and 1 micronM cyclic AMP in the assay mixture. In the presence of 2.5 X 10(-5)M L-triiodothyronine (T3), protein kinase activity was significantly reduced; 50% inhibition was achieved at 1 X 10(-4) M. Tests of diverse thyroid hormone analogs showed that T3 and its derivatives were more potent inhibitors than T4 and its derivatives which, in turn, were more potent than thyronine or diiodothyronine. Mono- and diiodotyrosine, tyrosine, and iodide were without effect. Triiodothyronine did not inhibit kidney, spleen, or lung protein kinase activity. The magnitude of the inhibition was the same whether or not cyclic AMP (1 micronM) was present in the incubation mixture, suggesting an effect on the catalytic, rather than the regulatory subunit of the enzyme. The inhibition of protein kinase by thyroid hormone was not influenced by Mg++ concentration but was overcome in a competitive manner by increasing ATP concentration. Increasing the histone concentration did not modify the inhibition. Although these studies suggest a novel cellular control mechanism, the high thyroid hormone concentrations required and the lack of concordance between inhibitory effects and biologic activity of the analogs tested precludes assumption of physiologic relevance.

Regulation of thyroid ornithine decarboxylase by the polyamines. Induction of a protein inhibitor of ornithine decarboxylase by the end-products of the reaction

When spermidine, putrescine or 1,3-diaminopropane was injected (12.5 mumol/100 g body weight) into rats 1 h before thyrotropin, ornithine decarboxylase activity was increased by 75--150% over control levels. However, when greater than or equal to 75 mumol polyamine/100 g body weight was injected, thyrotropin-activated activity was inhibited by 70--95%. Multiple polyamine injections inhibited goitrogen-induced activity and gland weight increase by approx 35%. The polyamines also inhibited thyrotropin-activated rat thyroid ornithine decarboxylase in vitro in a dose-related fashion, with 50% inhibition occurring at 2--5 . 10(-4)M. The inhibition was not due to a direct effect on the enzyme. No stimulation was seen with low concentrations of polyamine. The polyamines had no effect on in vitro thyroid protein/RNA synthesis or glucose oxidation but had a biphasic effect on plasma membrane adenylate cyclase activity. A protein inhibitor to thyroid ornithine decarboxylase was generated in vivo by multiple injections of the polyamines into rats and in vitro by incubating bovine thyroid slices with 2--10 mM polyamine. The inhibitor was non-dialyzable, destroyed by boiling, and its formation was blocked in a dose-related fashion by cycloheximide. We conclude that: (1) thyroid ornithine decarboxylase is subject not only to positive control, but is also negatively regulated by its end-products, the polyamines, which induce a protein inhibitor to ornithine decarboxylase; (2) since gland growth is also inhibited under these conditions, the polyamine effect on thyroid ornithine decarboxylase may be biologically significant.

Characteristics of thyrotropin binding to bovine thyroid plasma membranes and the influence of human IgG

The characteristics of binding of 125I-labeled bovine thyrotropin to bovine thyroid plasma membranes were studied. At pH 7.5, 0 C, specific binding was maximal in 3 hr and was progressively inhibited by Na+, Mg++, Ca++ and (NH4)2 SO4 in concentrations of 25 mM and greater. Affinity constants of l.047 X 10(8)M-1 and 0.57 X 10(6)M-1 were obtained for two binding sites and Hill plots provided a mean slope of 0.86, suggesting negative cooperativity. Addition of 1-thyroxine or 1-triiodothyronine at 0.5 ng to 50 microgram per ml had no significant effect but propylthiouracil (10(-7) to 10(-5)M) and KC104 (10(-7) to 10(-4)M) had biphasic effects, first enhancing and then inhibiting binding. Normal human serum IgG, particularly two fractions (3 and 4) obtained by chromatography on columns of cellulose phosphate, and an alpha-globulin - rich fraction from chromatography of serum proteins on diethylaminoethyl cellulose, inhibited binding at concentrations of 0.5 mg per ml or greater. Thus two possibilities are that normal human serum globulins either compete with thyrotropin for its receptor on thyroid plasma membranes or, in binding to the membranes, cause such perturbation as to affect adversely the conformation of the receptor for thyrotropin.

Effects of thyrotropin and thyroid hormones in vivo on thyroid responsiveness to thyrotropin in vitro

The thyroid gland of rats fed propylthiouracil is known to be unresponsive in vitro to thyrotropin; to investigate further the underlying mechanism groups of rats were variously treated with propylthiouracil and thyroid hormone or subjected to hypophysectomy. In vitro responsiveness of the thyroids was tested by measuring an increase in the concentration of c AMP when thyrotropin or prostaglandin E1 was added to the medium. Results showed that responsiveness to thyrotropin partially returned with rats fed prophylthiouracil and hypophysectomized 5, but not 2, days before death; hypophysectomy of normal rats led to increased in vitro responsiveness to thyrotropin and this was partially reversed by injections of thyrotropin for a week before death. Administration of thyroid hormone had little effect in these investigations and in vitro responsiveness to prostaglanding E1 was not consistently influenced by any of the in vivo regimens. From this experience we conclude that, at least as studied in vitro, circulating thyrotropin has a significant role in modulating responsiveness of the thyroid to thyrotropin.