Type 1 and type 2 iodothyronine deiodinases in the thyroid gland of patients with huge goitrous Hashimoto's thyroiditis.

PURPOSE:The serum free triiodothyronine (FT₃)/free thyroxine (FT₄) ratio in patients with huge goitrous Hashimoto's thyroiditis (HG-HT) is relatively high. We investigated the cause of high FT₃/FT₄ ratios. METHODS:We measured the serum FT₃, FT₄, and thyrotropin (TSH) levels of seven patients with HG-HT who had undergone a total thyroidectomy. Eleven patients with papillary thyroid carcinoma served as controls. The activities and mRNA levels of type 1 and type 2 iodothyronine deiodinases (D1 and D2, respectively) were measured in the thyroid tissues of HG-HT and perinodular thyroid tissues of papillary thyroid carcinoma. RESULTS:The TSH levels in the HG-HT group were not significantly different from those of the controls. The FT₄ levels in the HG-HT group were significantly lower than those of the controls, whereas the FT₃ levels and FT₃/FT₄ ratios were significantly higher in the HG-HT group. The FT₃/FT₄ ratios in the HG-HT group who had undergone total thyroidectomy and received levothyroxine therapy decreased significantly to normal values. Both the D1 and D2 activities in the thyroid tissues of the HG-HT patients were significantly higher than those of the controls. However, the mRNA levels of both D1 and D2 in the HG-HT patients' thyroid tissues were comparable to those of the controls. Interestingly, there were significant correlations between the HG-HT patients' D1 and D2 activities, and their thyroid gland volume or their FT₃/FT₄ ratios. CONCLUSIONS:Our results indicate that increased thyroidal D1 and D2 activities may be responsible for the higher serum FT₃/FT₄ ratio in patients with HG-HT.

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