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 μg 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 μg 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 T₃ 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.