3,5,3,'-Triiodothyroacetic acid (Triac) has been used in therapy of resistance to thyroid hormone on an empirical basis and appears beneficial in some studies. We observed that the T3 analogs, Triac and 3,5,3'-triiodothyropropionic acid (Triprop), have a higher affinity for the thyroid hormone receptor-beta 1 (TR beta 1) than does T3 (2.7- and 1.8-fold, respectively), whereas the affinities of the three compounds for TR alpha 1 are the same. To evaluate whether T3 analogs would have a differential effect on TR beta 1 and TR beta 1 mutants and thus be a specific treatment for patients with resistance to thyroid hormone, we examined the induction of the transcriptional activation of wild-type (wt) TR alpha 1, TR beta 1, and mutant TR beta 1s by T3, Triac, and Triprop. The dose response of transcriptional activation by T3 analogs was measured by transient cotransfections with TRs and a rat malic enzyme-TRE fused to thymidine kinase (TK)-chloramphenicol acetyltransferase (CAT) in COS-1 cells. For TR alpha 1 wt, induction of CAT activity by T3 and Triac occurred at the same concentration. For TR beta 1 wt, Triac and Triprop showed a higher maximal activity than T3 (Tripro > Triac > T3) and reached 50% induction at a lower concentration than T3 (Tripro < Triac < T3). Induction of CAT activity in five mutant TR beta 1s (kindreds Mh, Mc, CL, Mf, and GH) was also analyzed. Even high levels of T3 analogs could not restore CAT activity to that of TR beta 1wt for any mutant. A dominant negative effect was produced by Mh, Mc, and Mf. Mutants CL and GH had a mild dominant negative effect depending on T3 analog concentrations and TREs. Cotransfection studies were performed using a rat malic enzyme-TK-CAT reporter plasmid to analyze the effects of hormones at near-physiological concentrations of T3 and Triac. Triac had a significantly higher transcriptional activation than T3 in Mc, CL, and GH, suggesting that Triac would have a beneficial effect to different degrees for different mutant TR beta 1s. Using mutants Mc and GH, further studies were carried out using rat GH and double palindromic and inverted palindromic TREs in COS-1 cells. On each TRE, 10 nmol/L Triac induced higher transcriptional activation in TR beta 1wt, mutant TR beta 1s, and TR beta 1wt plus mutant TR beta 1s (1:1 ratio) than the same dose of T3.(ABSTRACT TRUNCATED AT 400 WORDS)