Differences in Response of Thyrotropin to 3,5,3′-Triiodothyronine and 3,5,3′-Triiodothyroacetic Acid in Patients with Resistance to Thyroid Hormone

This study investigated the response of TSH secretion to 3,5,3′-triiodothyronine (T3) and 3,5,3′-triiodothyroacetic acid (Triac) in patients with resistance to thyroid hormone, and compared the responses with those in patients with TSH-secreting pituitary adenoma and normal subjects. A short-term administration of 75 μg of T3 daily for 7 days suppressed serum TSH concentrations almost completely in normal subjects, but suppressed TSH only partially in patients with resistance to thyroid hormone and TSH-secreting pituitary adenoma. A single-dose administration of 75 μg of T3 gave similar results in regard to TSH suppressibility in these three subjects groups. In contrast, a single-dose administration of 1.4 mg of Triac remarkably suppressed serum TSH concentrations after 2 hours in not only normal subjects (-34 ± 11% [mean ± SD] from the basal value) but also in patients with resistance to thyroid hormone (-31 ± 9%), and this TSH suppression continued for 4 hours. After 24 hours, this TSH suppression persi...