STUDIES ON THE PITUITARY-THYROID AXIS IN HASHIMOTO'S THYROIDITIS

Abstract

Previous studies indicate that transiently decreased pituitary reserve follows withdrawal of suppressive thyroid therapy from intrinsically euthyroid patients, regardless of whether they are normal or goitrous. However, little is known of such TSH secretion patterns in Hashimoto's thyroiditis. Studies were conducted in 24 women patients with Hashimoto's thyroiditis. Twelve patients with thyroid adenoma served as control (Group I). Patients with Hashimoto's thyroiditis were classified as Group II (≤10μU/ml) and Group III (>10μU/ml) on the basis of serum TSH level 8 weeks after withdrawal of the exogenous thyroid hormone. They had no signs of hypothyroidism before therapy (euthyroid Hashimoto's thyroiditis) and received either L-thyroxine (L-T4, 200μg/day) or L-triiodo thyronine (L-T3, 50μg/day) for varying periods (1.5-60 months) L-T3 withdrawal : Before L-T3 taking off, serum levels of T4-I, T3 and TSH in Group II and Group III were not significantly different from those of Group I. No detectable TSH response to TRH was found in all groups. At the first week after withdrawal of suppressive medication, serum T3 decreased abruptly, while serum T4-I tended to increase. Thereafter, serum T4-I and T3 rose progressively, returning into the normal ranges by the second week, although the values of Group III appeared to be lower than those of Group I and H. The maximal basal TSH level was obtained in Group II and Group ill at the first week, followed by further decreases by the 8th week. By contrast, no change was observed in the basal serum TSH in Group I. There was an exaggerated response of pituitary TSH to TRH (500μg, iv) in Group II and III through 1-8 weeks. The higher serum TSH level was observed in Group III than in Group II.L-T4 withdrawal : There was no change in serum levels of T4-I, T3 and TSH during L-T4 therapy. A rapid fall in serum T4-I and T3 occurred at the first week after withdrawal of L-T4, followed by a further decline to nadir at the second week. The supranormal serum TSH and exaggerated TSH response to TRH were observed at 4-8 weeks in Group II and III. The higher TSH level was also obtained in Group III than in Group II.On the other hand, no difference in the basal serum prolactin (PRL) and TRH-stimulated PRL response between Hashimoto's thyroiditis and thyroid adenoma was observed through the study.The present investigation has shown that basal serum TSH level can differentiate thyroid adenoma from Hashimoto's thyroiditis 1 week for L-T3 and 2 weeks for L-T4 after withdrawal of thyroid therapy. Serum TSH response to TRH does not potentiate this differentiation. In addition, differential sensitivity of the lactotrophs and thyrotrophs in Hashimoto's thyroiditis following discontinuing thyroid medication was also demonstrated.