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

OBJECTIVES: The 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 METHODS: We 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.

RESULTS: The 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.

CONCLUSION: Our results suggest that the decreased serum T3 is a major cause of impaired TSH-thyroid hormone feedback control in hypothyroidism treated with LT4.