Modulation of Pituitary Responsiveness to Throtropin-Releasing Hormone by Triiodothyronine

Abstract

A B S T R A C T The relative roles of triiodothyronine (T3) and thyroxine (T4)

Propylthiouracil blocks extrathyroidal conversion of thyroxine to triiodothyronine and augments thyrotropin secretion in man.

“…It occurred at a time when there was no significant change in serum T4 concentration. Previous investigators have shown that PTU administration increased the amount of bioassayable TSH in the serum of thyroidectomized to TRH at a time when in some patients serum T4 is elevated but T3 has fallen to normal concentrations (42). Our findings are consistent with these observations.…”

Suppression of Pituitary TSH Secretion in the Patient with a Hyperfunctioning Thyroid Nodule

“…These data are consistent with those of Snyder and Utiger (40) who have demonstrated the extreme sensitivity of the TRH-induced TSH release to inhibition in normal and hypothyroid subjects after the chronic oral administration of quantities of T4 and T'3 that did not raise serum T4D and T3(RIA) above the normal range. The sensitivity of the TRH-induced TSH release to inhibition by single doses (50-100 Ag) of T3 has also been reported (41,42). Thus it is evident that there is a very sensitive feedback control between the concentrations of circulating thyroid hormones and that of the secretion of TSH.…”

“…Basal serum TSH concentrations were undetectable (< 0.5 -1.0 AU/ ml) in all patients. The metabolic clearance of TSH in one patient before and after excision of the thyroid nodule was unchanged (40 vs. 42 ml/min) whereas the calculated production rate was undetectable before the operation (< 29 mU/day) and normal after (103 mU/ day). These data, in one patient, suggest that the undetectable concentration of TSH in these patients is a result of suppressed TSH secretion rather than accelerated TSH clearance.…”

Serum Thyrotropin Response to Thyrotropin Releasing Hormone and the Concentration of Thyroid Hormone in Patients with Hyperfunctioning Thyroid Nodule

“…Recently, synthetic thyrotropin-releasing hormone (TRH) has been used for the evaluation of TSH secretion in man. Lack of response of serum TSH to TRH administration has been shown in some thyroid disorders , such as diffuse toxic goiter (Kumahara et al, 1971;Lawton et al, 1971;Ormston et al, 1971;Shenkman et al, 1973) or subacute thyroiditis during acute phase (Ogihara et al, 1973). As an analogy of these thyroid disorders, Karlberg (1973) and Ridgway et al (1973) has demonstrated an absent TSH response to TRH in patients with single thyroid nodule, either in hyperthyroid or even in euthyroid state.…”

“…As an analogy of these thyroid disorders, Karlberg (1973) and Ridgway et al (1973) has demonstrated an absent TSH response to TRH in patients with single thyroid nodule, either in hyperthyroid or even in euthyroid state. Although triiodothyronine (T3) has been known to be an important modulator of responsiveness to TRH (Snyder and Utiger 1972a;Shenkman et al, 1973), serum T3 concentration has been found to be normal (Ridgway et al, 1973).…”