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

PURPOSE

The serum free triiodothyronine (FT)/free thyroxine (FT) ratio in patients with huge goitrous Hashimoto's thyroiditis (HG-HT) is relatively high. We investigated the cause of high FT/FT ratios.

METHODS

We measured the serum FT, FT, 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 FT levels in the HG-HT group were significantly lower than those of the controls, whereas the FT levels and FT/FT ratios were significantly higher in the HG-HT group. The FT/FT 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 FT/FT ratios.

CONCLUSIONS

Our results indicate that increased thyroidal D1 and D2 activities may be responsible for the higher serum FT/FT ratio in patients with HG-HT.