This study found that administration of T3 and T4 might be a more suitable replacement therapy than T4 alone.

Abstract

In order to investigate the effect of 3,5,3’L-triiodothyronine (T3) administration on thyroid hormone concentrations in serum, thyroxine (T4), T3, 3,3′,5’L-triiodothyronine (reverse T3, rT3) and thyroid stimulating hormone (TSH) concentrations in serum were determined before and after T3 administration in 10 hypothyroid patients maintained on constant doses of T4. Ten hypothyroid patients given 100 micrograms of T4 for approximately 3 months had almost normal T4 and T3 concentrations in serum. Seven patients showed almost normal rT3 concentrations in serum and they were slightly diminished in the remaining 3 patients. TSH levels in serum were almost within the normal limit in 7 out of 10 patients. However, despite the elevation of T4 and T3 levels, 3 patients had markedly elevated TSH levels. Values for serum T4 concentrations were decreased 4 weeks after the administration of 50 micrograms T3 in all patients maintained on constant doses of T4, although they were almost within the normal range. T3 concentrations in serum, which was obtained just before the administration of the next daily doses of T3, were markedly elevated in 6 of 10 patients after T3 administration and the remaining 4 patients had also slightly higher T3 concentrations than those before T3 administration. On the other hand, serum rT3 concentrations were diminished in 5 patients during T3 ingestion. They were somewhat diminished or almost unchanged before and after T3 administration in the remai T3 administration and the remaining 4 patients had also slightly higher T3 concentrations than those before T3 administration. On the other hand, serum rT3 concentrations were diminished in 5 patients during T3 ingestion. They were somewhat diminished or almost unchanged before and after T3 administration in the remaining 5 patients. Moreover, 3 patients with elevated TSH levels during T4 administration showed almost normal TSH levels after T4 and T3 ingestion. The results showed the reciprocal relationship between T3 and rT3 levels in serum after T3 administration in hypothyroid patients maintained on constant doses of T4. Furthermore, the present findings suggest that the administration of both T4 and T3 might be a more suitable replacement therapy in the patients with hypothyroidism than T4 alone.

http://www.ncbi.nlm.nih.gov/pubmed/7439119