Effect of 3,5,3'L-triiodothyronine administration on serum thyroid hormone levels in hypothyroid patients maintained on constant doses of thyroxine.

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Abstract
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 thyrotropin, (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 .MU.g T4 for .apprx. 3 mo. 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 of 10 patients. Despite the elevation of T4 and T3 levels, 3 patients had markedly elevated TSH levels. Values for serum T4 concentrations were decreased 4 wk after the administration of 50 .mu.g T3 in all patients maintained on constant doses of T4, but 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 also had slightly higher T3 concentrations than those before T3 administration. 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. Three patients with elevated TSH levels during 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. The administration of both T4 and T3 may be a more suitable replacement therapy in the patients with hypothyroidism than T4 alone.

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