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 microg 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 microg 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 persisted in normal subjects (-62 +/- 12%) but was relieved in 
patients with resistance to thyroid hormone (-23 +/- 14%). After the Triac 
administration, molar ratios of alpha-subunit to TSH in serum were decreased in 
patients with TSH-secreting pituitary adenoma but increased in patients with 
resistance to thyroid hormone. Because the Triac therapy for patients with 
resistance to thyroid hormone suppressed pituitary-TSH secretion during the 
early phase of drug ingestion, this drug should be given several times within a 
day to obtain continuous TSH-suppressive effects.