DIFFERENCES IN RESPONSE OF THYROTROPIN TO 3,5,3'-TRIIODOTHYRONINE AND3,5,3'-TRIIODOTHYROACETIC ACID IN PATIENTS WITH RESISTANCE TO THYROID-HORMONE

This study investigated the response of TSH secretion to 3,5,3'-triiod othyronine (T-3) and 3,5,3'-triiodothyroacetic acid (Triac) in patient s with resistance to thyroid hormone, and compared the responses with those in patients with TSH-secreting pituitary adenoma and normal subj ects. A short-term administration of 75 mu g of T-3 daily for 7 days s uppressed serum TSH concentrations almost completely in normal subject s, but suppressed TSH only partially in patients With resistance to th yroid hormone and TSH-secreting pituitary adenoma. A single-dose admin istration of 75 mu g of T-3 gave similar results in regard to TSH supp ressibility in these three subjects groups. In contrast, a single-dose administration of 1.4 mg of Triac remarkably suppressed serum TSH con centrations after 2 hours in not only normal subjects (-34 +/- 11% [me an +/- SD] from the basal value) but also in patients with resistance to thyroid hormone (-31 +/- 9%), and this TSH suppression continued fo r 4 hours. After 24 hours, this TSH suppression persisted in normal su bjects (-62 +/- 12%) but was relieved in patients with resistance to t hyroid hormone (-23 +/- 14%). After the Triac administration, molar ra tios of alpha-subunit to TSH in serum were decreased in patients with TSH-secreting pituitary adenoma but increased in patients with resista nce to thyroid hormone. Because the Triac therapy for patients with re sistance to thyroid hormone suppressed pituitary-TSH secretion during the early phase of drug ingestion, this drug should be given several t imes within a day to obtain continuous TSH-suppressive effects.