VIP and hormone secretion from thyroidal follicular and C-cells

The effect of vasoactive intestinal peptide (VIP) on the cAMP system of the thyroid and on the secretion of T4 and T3 from the follicular cells and calcitonin and somatostatin from the C-cells was studied in perfused dog thyroid lobes. Activation of the cAMP system was evaluated by measurements of the amount of cAMP released into the perfusion medium. T4, T3, calcitonin and somatostatin were measured by radioimmunoassays. 3 X 10(-6) M VIP induced increases in cAMP release and T4 and T3 secretion from the thyroid while there were no significant alterations in calcitonin and somatostatin release (n = 4). In experiments employing both of the two isolated thyroid lobes 100 microU/ml TSH gave considerably higher increases in T4 and T3 secretion than 10(-6) M VIP (n = 4). The effect of 10(-9) M VIP on T4 and T3 secretion was similar to that of 10(-6) M VIP (n = 4). 10(-10) M VIP induced a small but statistically significant increase in T4 and T3 secretion in two experiments while no effect was observed in two dogs. This high sensitivity of the follicular cells to VIP and the demonstration by others of VIP containing nerves in the thyroid suggest that VIP-ergic nerves may be involved in the regulation of thyroid hormone secretion.     

Dynamics of hormone release from the perfused canine thyroid during cyclic AMP infusion.

In order to try to characterize the sequence of processes leading to hormone secretion from the stimulated thyroid, the effect of cyclic 3'5' adenosine monophosphate (cAMP) and related compounds were examined in 15 two-sided perfusions of canine thyroids isolated in situ. T4 and T3 concentrations in the effluent were measured radioimmunologically. cAMP 5 mM and TSH 100 muU per ml induced the same pattern of hormone release from the thyroid. After a latency period of 15--25 minutes a steep increase occurred in both T4 and T3 release. During the initial part of the stimulation the rise in T4 relase was somewhat slower than that of T3 release. The prolonged latency period before response earlier recorded in the same preparation during infusions of low concentrations of TSH was not observed during infusions of decreasing concentrations of cAMP (1, 0.8, 0.5 and 0.2 mM) or theophylline (5 and 1 mM). Either there was no response or the latency period was of the same length as that observed after a strong stimulus. These findings suggest that the latency period can be divided in two parts: () a variable, dose dependent satency period confined to the early part of the process sequence leading to secretion--i.e. before cAMP exerts its effect, and 2) an obligatory latency period related to the processes taking place after the formation of pseudopods. The duration of these late processes seems to be independent of the degree of stimulation.     

Direct effects of trypan blue on thyroid secretion.

Trypan blue directly inhibited in vitro thyroid secretion (butanol soluble 125I release to the media) induced by both thyroid stimulating hormone (TSH) and dibutyryl cAMP. Intracellular colloid droplet counts were also decreased. Inhibition was directly proportional to dye concentration and could be overcome by supramaximal TSH and dibutyryl cAMP. Inhibition could be observed as early as 20 min of incubation, was not increased by preincubation, and could even be demonstrated after TSH in vivo. Trypan blue, in vivo, produced similar inhibition of thyroid secretion. Incubation of 125I-thyroglobulin with lysosomal enzymes revealed inhibition with much lower concentrations of dye. Inhibition of lysosomal enzyme(s) would not appear to explain the marked decreases in colloid droplets, and this may represent two separate effects of trypan blue on thyroid secretion.     

Epidermal growth factor (EGF) inhibits stimulated thyroid hormone secretion in the mouse

It is known that epidermal growth factor (EGF) inhibits iodide uptake in the thyroid follicular cells and lowers plasma levels of thyroid hormones upon infusion into sheep and ewes. In this study, the effects of EGF on basal and stimulated thyroid hormone secretion were investigated in the mouse. Mice were pretreated with 125I and thyroxine; the subsequent release of 125I is an estimation of thyroid hormone secretion. It was found that basal radioiodine secretion was not altered by intravenous injection of EGF (5 micrograms/animal). However, the radioiodine secretion stimulated by both TSH (120 microU/animal) and vasoactive intestinal peptide (VIP; 5 micrograms/animal) were inhibited by EGF (5 micrograms/animal). At a lower dose level (0.5 microgram/animal), EGF had no influence on stimulated radioiodine secretion. In conclusion, EGF inhibits stimulated thyroid hormone secretion in the mouse.     

Highly sensitive immunoradiometric assay for TSH and thyroid radioiodine uptake as predictors of thyroid suppression test

Suppression of TSH and thyroid radioiodine uptake by doses of either T4 or T3 were compared in 33 patients in whom Graves' thyrotoxicosis had been treated with thioamide drugs and the medication was discontinued for at least 4 months. Thyroidal radiodine uptake was suppressed in 19 patients and was not suppressed in the remaining 14 patients. Basal TSH levels before suppression were 2.07 microU/ml in the former, significantly exceeding those of the latter (0.91 microU/ml). A TSH level of at least 1.2 microU/ml before suppression is a good predictor of positive thyroid radioiodine suppression with a predictive value of 76%. A level lower than 0.7 microU/ml before suppression is a good predictor of negative thyroid radioiodine uptake suppression with a predictive value of 89%. The determination of TSH levels before the thyroid suppression test was helpful in predicting the result, but there were limitations. In the thyroid suppression test positive group, circulating T4 was depressed by doses of T3. In them, the magnitude of T4 depression correlated with the levels of thyroid radioiodine uptake before suppression. The levels of TSH correlated neither to changes in T4 nor to those in thyroid radioiodine uptake. This indicates that the thyroid glands which show high radioiodine uptake are sensitive to TSH and are also sensitive to suppression. The elevated sensitivity to TSH probably warrants the disappearance of abnormal thyroid stimulation more precisely.     

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.     

Effect of thyrotropic hormone on the membrane potential of thyroid gland cells and thyroid hormone secretion with aging

Effect of thyrotropic hormone (TTH) on membrane potential (MP) of thyroid cells and thyroid hormone secretion was studied in experiments on male rats of two age groups (7--12- and 27--32-month-old animals). It was found that during the first 3 hours after TTH administration (5 U/100 g i. v.) the depolarization of secretory cell membranes of adult rats was done pronounced and developed more rapidly than in old ones and that an increase in free thyroxin (T4) correlation with MP changes with time. In a dose of 0.5 U/100 g TTH caused a significant rise in T4 secretion only in old rats. The cAMP level in the thyroid gland declined with aging. In a dose of 5 U/100 g TTH provoked a significant increase in the cAMP content in adult rats and had no effect on its content in old ones. A relationship between the MP level of thyroid secretory cells and thyroid hormone secretion is discussed.     

In vitro study on release of thyroid hormone in solitary autonomously functioning thyroid nodules using cell culture method

In vitro release of thyroid hormone was investigated under basal and TSH-stimulated conditions in the solitary autonomously functioning thyroid nodules (AFTN). A small portion (0.5 g of wet weight) of the nodules and adjacent thyroid tissues removed surgically from five patients with solitary AFTN were prepared for the dispersed cell culture. In the experiment on non TSH-stimulated (basal) conditions, those culture media which were totally replaced on the 5th day after primary culture were utilized for the determination of thyroxine (T4) and triiodothyronine (T3) by radioimmunoassay. T4 and T3 levels in culture media of the functioning nodules were 1.15 +/- 0.33 microgram/dl (mean +/- SEM) and 2.72 +/- 0.68 ng/ml, contrasted with levels of 0.67 +/- 0.09 microgram/dl and 1.24 +/- 0.22 ng/ml in the paranodular tissues. The mean ratios of T3/T4 of the nodules and paranodular tissues were 0.25 +/- 0.02 and 0.19 +/- 0.02, respectively (p less than 0.05). Meanwhile, in another experiment under TSH stimulatory conditions employing 40 and 80 microU/ml of human TSH, there were no significant differences in T4 and T3 releases when the two groups were compared.     

Chronic effect of TSH on human thyroid tissue in organ culture

The chronic effect of TSH on thyroidal cAMP concentrations and release of thyroid hormones was investigated using human thyroid tissue in organ culture. Normal human thyroid slices were placed in HAM's F-10 synthetic culture medium in Falcon organ tissue culture dishes, and incubated at 37 degrees in a humidified atmosphere of 5% CO2 in air. Medium was changed everyday and daily T3 or T4 release was determined using concentration of T3 or T4 in the medium. After incubation, slices were transferred to the medium containing 10 mM theophylline and incubated without TSH for an additional 30 min to determine thyroidal cAMP concentrations. Thyroidal cAMP concentrations in slices incubated with 10 mU/ml of TSH increased significantly at 2, 6, and 24 hr and even on the 6th day of incubation. Daily T3 release was significantly increased above control from the 3rd day and daily T4 release from the 4th day to the 11th day of incubation with 10 mU/ml of TSH. Histologically, almost all follicles were structurally maintained even on the 11th day of incubation. These results suggest that both thyroidal cAMP concentrations and release of thyroid hormones are stimulated chronically by TSH. This organ culture system is useful for investigating chronic effects of various materials on human thyroid tissue.     

The thyroid reserve in children with chronic lymphocytic thyroiditis revealed by the thyrotropin-releasing hormone and thyroid-stimulating hormone tests

Serum thyroid hormone and TSH concentrations were measured before and after the administration of TRH (10 micrograms/kg body weight) and bovine TSH (10 IU) in 14 children with chronic lymphocytic thyroiditis. The TRH test showed that the responsiveness of TSH was positively correlated with the basal TSH (P less than 0.001) and inversely with the increase in serum thyroid hormones, for delta T3 (P less than 0.05) and for delta T4 (P less than 0.001). Overall, the patients had significantly lower mean values for basal T4, but not for T3. The TSH test revealed that the delta T3 was positively correlated with delta T4 (P less than 0.05). delta T3 after TSH administration was positively correlated with it after TRH (P less than 0.05). The patients were divided into three groups on the basis of their peak TSH values after TRH administration. In Group 1 (peak value below 40 microU/ml; N = 5); T3 increased significantly after TRH and TSH administrations (P less than 0.05 and P less than 0.025, respectively). In addition, delta T4 was significant after TSH administration. In Group 2 (peak TSH above 40 and less than 100 microU/ml; N = 6); only delta T3 after TRH was significant (P less than 0.05). In Group 3 (peak TSH above 100 microU/ml; N = 3); the response of thyroid hormones was blunted. Thus, the thyroid hormone responses to endogenous TSH coincided with that to exogenous TSH, and the exaggerated TSH response to TRH indicates decreased thyroid reserve.     

The inhibitory effects of calmodulin antagonists on TSH-stimulated thyroid hormone release by mouse thyroid lobes

Calcium ions have been shown to play a mojor regulatory role in the release of various hormones from a wide variety of endocrine organs. More recently, in vitro evidence suggests that a calcium-binding protein, calmodulin, is also involved in the release of many hormones. So we examined the effects of several types of calmodulin antagonists on TSH-stimulated thyroid hormone release in vitro. Mouse thyroid lobes (one thyro-tracheal unit/tube) were incubated in Krebs-Ringer bicarbonate buffer at 37 degrees C for 4h. Free thyroxine (fT4) released in the incubation medium, thyroidal cAMP and calmodulin content were measured by RIA. TSH (5 mU/ml) and dibutyryl cAMP (DBC) (200 micrograms/ml) caused a 2-4 fold increase in thyroidal release of fT4. The stimulatory effects of TSH on fT4 release were significantly inhibited by trifluoprazine and prenylamine lactate at the concentration of 5 X 10(-5) M. More specific calmodulin antagonists, W-7 and W-13, were also shown to inhibit TSH stimulation of fT4 release at the concentration of 5 X 10(-5) M. In contrast, TSH stimulation of fT4 release was not depressed by non-specific antagonists, W-5 or W-12, at the same concentration as 5 X 10(-5) M. Further, W-13 also markedly inhibited DBC-stimulated fT4 release. Neither TSH nor PGI2 altered the thyroidal calmodulin content, dissociating with a marked increase in the cAMP concentration. These results suggest that calmodulin plays an important role in TSH-stimulated thyroid hormone release and further that this mechanism exists, at least in part, at the site subsequent to the generation of cAMP.     

Maturation of the pituitary-thyroid axis during the perinatal period.

To clarify the maturation process of the pituitary-thyroid axis during the perinatal period, thyrotropin (TSH) response to thyrotropin releasing hormone (TRH) and serum thyroid hormone levels were examined in 26 healthy infants of 30 to 40 weeks gestation. A TRH stimulation test was performed on 10 to 20 postnatal days. Basal concentrations of serum thyroxine (T4), free thyroxine (free T4) and triiodothyronine (T3) were positively correlated to gestational age and birth weight (p less than 0.001-0.01). Seven infants of 30 to 35 gestational weeks demonstrated an exaggerated TSH response to TRH (49.7 +/- 6.7 microU/ml versus 22.1 +/- 4.8 microU/ml, p less than 0.001), which was gradually reduced with gestational age and normalized after 37 weeks gestation. A similar decrease in TSH responsiveness to TRH was also observed longitudinally in all of 5 high responders repeatedly examined. There was a negative correlation between basal or peak TSH concentrations and postconceptional age in high responders (r = -0.59 p less than 0.05, r = -0.66 p less than 0.01), whereas in the normal responders TSH response, remained at a constant level during 31 to 43 postconceptional weeks. On the other hand, there was no correlation between basal or peak TSH levels and serum thyroid hormones. These results indicate that (1) maturation of the pituitary-thyroid axis is intrinsically controlled by gestational age rather than by serum thyroid hormone levels, (2) hypersecretion of TSH in preterm infants induces a progressive increase in serum thyroid hormones, and (3) although there is individual variation in the maturation process, the feedback regulation of the pituitary-thyroid axis matures by approximately the 37th gestational week.     

Relation of structural polarity to responses of cyclic 3', 5'-adenosine monophosphate accumulation and thyroid hormone release in cultured human thyroid follicles

The relationship of structural polarity to functional activities was examined in cultured human thyroid follicles, which were isolated from the thyroid gland of patients with Graves' disease by collagenase treatment. Structural polarity was examined morphologically by electron microscopy, while the functional response to bovine TSH was examined by measuring intracellular cAMP accumulation and T3 release. In freshly isolated thyroid follicles, structural polarity was normal and TSH induced significant cAMP accumulation but no significant release of T3. After culture for 5 days the structural polarity of thyroid follicles became inverted in the absence of thyroid stimulators, but normal polarity was retained in the presence of TSH or dibutyryl cAMP [Bu)2 cAMP). The response to TSH of cAMP accumulation increased markedly after culture in either the presence or absence of TSH, suggesting that cAMP accumulation in response to TSH is not related to structural polarity. In contrast, thyroid follicles cultured without thyroid stimulators showed no significant T3 release in response to TSH, whereas those cultured with TSH or (Bu)2 cAMP showed significant T3 release in response to TSH. These data indicate that in cultured human thyroid follicles, the responses to TSH of cAMP accumulation and T3 release are not always correlated. Among many other explanations, the results were at least compatible with the idea that normal structural polarity is necessary for thyroid hormone release in response to TSH.     

Abnormalities of the thyroid hormone negative feedback regulation of TSH secretion in spontaneously hypertensive rats.

Spontaneously hypertensive rats (SHR) are characterized by several neuroendocrine abnormalities including a chronic hypersecretion of thyrotropin (TSH) of unknown etiology. We hypothesized that the inappropriately high TSH secretion in SHR may be the result of an impaired thyroid hormone negative feedback regulation of hypothalamic thyrotropin-releasing hormone (TRH) and/or pituitary TSH production. To test this hypothesis, SHR or their normotensive Wistar-Kyoto (WKY) controls were treated with either methimazole (0.02% in drinking water) to induce hypothyroidism or administered L-thyroxine (T4) at a dose of 0.8 or 2.0 micrograms/100 g body weight/day to induce hyperthyroidism. All treatments were continued for 14 days after which animals were killed under low stress conditions. TSH concentrations in plasma and anterior pituitary tissue were 2-fold higher (P less than 0.01) in euthyroid SHR compared to WKY control rats while thyroid hormone (T3 and T4) levels were in the normal range. Hypothyroidism induced by either methimazole or thyroidectomy caused a significant (P less than 0.01) rise of plasma TSH levels in both WKY and SHR rats. However, relative to the TSH concentrations in control animals, the increase of plasma TSH in SHR was significantly blunted (P less than 0.01) in comparison to the WKY group. Hypothyroidism caused a significant depletion of TRH in stalk-median eminence (SME) tissue in both groups of rats. However, no differences between SHR and WKY rats were observed. The administration of thyroid hormone caused a dose dependent suppression of plasma TSH levels in both strains of rats. However, at both doses tested plasma TSH concentrations in SHR rats were significantly less suppressed (P less than 0.05) than those in WKY animals. Under in vitro conditions basal and potassium induced TRH release from SMEs derived from SHR was significantly (P less than 0.05) higher than that from WKY rats, whether expressed in absolute terms or as percent of content. These findings suggest that the thyroid hormone negative feedback regulation of TSH secretion may be impaired in SHR rats. Our data do not allow conclusions as to whether defects in the regulation of TSH production are located exclusively at the hypothalamic level. Since the overproduction of hypothalamic TRH and hypophysial TSH should lead to an increased thyroid hormone biosynthesis other defects in the hypothalamus-pituitary-thyroid-axis may contribute to the abnormal regulation of TSH secretion in SHR rats.     

Changes in thyrotroph and somatotroph cell populations induced by stimulation and inhibition of their secretory activity

Summary The populations of cells which produce immunoreactive growth hormone (GH) and thyroid stimulating hormone (TSH) in the rat pituitary gland do not occur in fixed percentages but vary greatly under different physiological and experimental conditions. These variations can be directly correlated to the levels of stimulation and/or inhibition of the specific secretory activity. In both types of cell, sustained stimulation with trophic hormones or blockage of the feedback mechanisms induces remarkable growth in the specific cell population. Conversely, the interruption or inhibition of the stimulus thwarted the hormonal secretion and caused a massive degeneration of redundant cells. The stimulation of both GH and TSH cells is accompanied by an enhanced secretory activity as judged by their higher concentrations in serum and hypertrophy of the cytoplasmic organelles involved in synthesis and intracellular processing of the hormones. By contrast, interruption of the stimulus is followed by a variable degree of disruption of the cytoplasmic organization, including a sizable degeneration of cells. In stimulated rats, the concentrations of both GH and TSH decreased significantly in pituitary tissue due to mobilization of the hormonal stores contained in secretory granules. On the other hand, the withdrawal of stimuli blocked the hormonal release; this is reflected by the accumulation of both hormones and secretory granules in pituitary tissue. The strict correlation between the size of the GH and TSH populations with stimulation and inhibition of hormonal secretory activity reported in this investigation further supports the critical role played by the cell renewal process in endocrine secretion.     

Thyrotropin receptor non-mediated thyroid stimulating immunoglobulin in Graves' disease.

There exists a consensus that hyperthyroid Graves' disease is caused by thyrotropin receptor (TSH-R) autoantibodies. To test the possibility that the TSH-R is the sole antigen for thyroid stimulating antibodies (TSAb), we compared bioactivities of Graves' IgGs between non-thyroid mammalian cells transfected with human TSH-R cDNA and the reference thyroid bioassay. A Graves' IgG with TSH-binding inhibitor immunoglobulin (TBII) activity (89%) markedly stimulated cAMP formation in both CHO-K1 cells transfected with TSH-R cDNA (340 microU/ml of TSH equivalent) and rat thyroid cells, FRTL-5, (410 microU/ml of TSH equivalent). In contrast, a TBII negative (-1.5%) IgG from another patient with Graves' disease showed a strong thyroid stimulating activity (87 microU/ml of TSH equivalent) when FRTL-5 cells were used for the assay. But no stimulating activity was observed in this IgG when CHO-K1 cells transfected with TSH-R cDNA were used, suggesting a possible existence of TSH-R non-mediated thyroid stimulating immunoglobulin in some cases of Graves' disease.     

Maturation of feedback control of thyrotropin in premature infants

Serum thyrotropin (TSH), free T4 and free T3 concentrations were measured longitudinally in 26 preterm infants for 14 weeks after birth, using highly sensitive immunoradiometric assays. Serum TSH values on days 4-5 were positively correlated with gestational age and birth weight. In the premature infants of 25 weeks mean gestation, the mean TSH concentrations increased from a very low value of 0.84 microU/ml at 5 days to a peak value of 6.1 microU/ml by 5 weeks of age, then slightly decreased and remained stable. Serum free T4 and free T3 concentrations increased in parallel and free T3 level reached the range of term infants by 6 weeks. Serum free T4/TSH and free T3/TSH ratios began to increase at the 6th week of age. The results suggest that: (i) the thyroid hormone feedback control of pituitary TSH release in the extremely premature infants begins to mature after 6 weeks of postnatal age, (ii) the maturation pattern of the hypothalamic-pituitary-thyroid system in premature infants is similar to that of the intrauterine fetus.     

Thyroid stimulating hormone upregulates secretion of cathepsin B from thyroid epithelial cells

Constant levels of thyroid hormones in the blood are principal requirements for normal vertebrate development. Their release depends on the regulated proteolysis of thyroglobulin which is extracellularly stored in the follicle lumen under resting conditions. Thyroglobulin is proteolytically degraded to a major part in lysosomes, but in part also extracellularly leading to the release of thyroxine. Extracellularly occurring lysosomal enzymes are most probably involved in the proteolytic release of thyroxine. In this study we have analyzed the secretion of cathepsin B by thyroid follicle cells (primary cells as well as FRTL-5 cells) and its regulation by thyroid stimulating hormone, which stimulated the secretory release of the proenzyme as well as of mature cathepsin B. Within one to two hours of stimulation with thyroid stimulating hormone, the cathepsin B activity associated with the plasma membrane increased significantly. This increase correlated closely with the localization of lysosomes in close proximity to the plasma membrane of cultured thyrocytes as well as with the thyroxine liberating activity of thyrocyte secretion media. These observations indicate that thyroid stimulating hormone induces the secretion of cathepsin B, which contributes to the extracellular release of thyroxine by thyrocytes.     

Cholinergic and VIPergic effects on thyroid hormone secretion in the mouse

The thyroid gland is known to harbor cholinergic and VIPergic nerves. In the present study, the influences of cholinergic stimulation by carbachol, cholinergic blockade by methylatropine and stimulation with various VIP sequences on basal, TSH-induced and VIP-induced thyroid hormone section were investigated in vivo in mice. The mice were pretreated with 125I and thyroxine; the subsequent release of 125I is an estimation of thyroid hormone secretion. It was found that basal radioiodine secretion was inhibited by both carbachol and methylatropine. Furthermore, TSH-induced radioiodine secretion was inhibited already by a low dose of carbachol. Moreover, a high dose of carbachol could inhibit VIP-induced radioiodine secretion. Methylatropine did not influence TSH- or VIP-stimulated radioiodine secretion, but counteracted the inhibitory action of carbachol on TSH- and VIP-induced radioiodine release. In addition, contrary to VIP, six various synthesized VIP fragments had no effect on basal or stimulated radioiodine release. It is concluded that basal thyroid hormone secretion is inhibited by both cholinergic activation and blockade. Furthermore, TSH-induced thyroid hormone secretion is more sensitive to inhibition with cholinergic stimulation than is VIP-induced thyroid hormone secretion. In addition, the VIP stimulation of thyroid hormone secretion seems to require the full VIP sequence.     

Effect of the beta-adrenergic substances propranolol, alprenolol and isoprenaline on cAMP production in bovine thyroid slices

The effect of the beta-adrenergic blockers L-alprenolol and DL-propranolol and of the beta-adrenergic agonist L-isoprenaline on the basal and thyrotropic hormone(TSH)-stimulated cyclic adenosine-monophosphate (cAMP) level in bovine thyroid slices was studied. The main basal cAMP level in bovine thyroid slices was 3 pmol/mg tissue. TSH stimulated cAMP production in correlation to the concentration. Maximum stimulation was achieved with a TSH concentration of 10 mU/ml. The beta-blockers DL-propranolol and L-alprenolol caused 74 and 77% inhibition of TSH-stimulated cAMP synthesis respectively. The beta-adrenergic agonist L-isoprenaline did not significantly affect either the basal or the TSH-stimulated cAMP level. The role of the beta-adrenergic receptor system in the regulation of TSH-stimulated cAMP synthesis is discussed.