Decreased sensitivity of thyrotropin-thyroid hormone feedback control in hypothyroid and athyreotic patients treated with levothyroxine. Is serum triiodothyronine involved?

Abstract

ObjectivesThe search for optimal therapy for hypothyroidism is challenging and complex and is still going on. We investigated TSH-thyroid hormone feedback control in hypothyroid and athyreotic patients properly treated with levothyroxine (LT4), based on normal TSH values. Material and methodsWe randomly selected 150 hypothyroid, 110 athyreotic and 210 euthyroid subjects, determined their serum T4, T3 and TSH values and calculated T4/TSH and T3/T4 ratios. ResultsThe TSH distribution in hypothyroidism was significantly shifted to the higher and that in athyreosis to the lower values from the normal distribution in euthyroid controls. This TSH-thyroid hormone dissociation in hypothyroidism was characterized with higher T4 and higher matched TSH values. We found 5% hypothyroid and 10% athyreotic patients normalize TSH only with hyperthyroxinemia. Serum T3 was lower in both hypothyroid groups and unaffected by a higher LT4 dose and higher serum T4 in athyreosis. ConclusionOur results suggest that the decreased serum T3 is a major cause of impaired TSH-thyroid hormone feedback control in hypothyroidism treated with LT4.