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.
Keywords: Hashimoto’s thyroiditis; Iodothyronine deiodinase; Thyroid gland; Thyroid hormone metabolism.