Effect of the presence of remnant thyroid tissue on the serum thyroid hormone balance in thyroidectomized patients

Objective: We and others recently reported that in total thyroidectomy (TT), serum triiodothyronine (T-3) levels during levothyroxine (L-T-4) therapy were low compared to the preoperative levels, suggesting that the presence of the thyroid tissue affects the balances of serum thyroid hormone levels. However, the effects of remnant thyroid tissue on these balances in thyroidectomized patients have not been established. Methods: We retrospectively studied 253 euthyroid patients with papillary thyroid carcinoma who underwent a TT or hemithyroidectomy (HT). We divided the cases into the TT+supplemental L-T-4 (+L-T-4) group (n=103); the HT+L-T-4 group (n=56); and the HT-alone group (n=94). We compared the postoperative serum levels of free T-4 (FT4) and free T-3 (FT3) and the FT3/FT4 ratio in individual patients with those of controls matched by serum TSH levels. Results: The TT+L-T-4 group had significantly higher FT4 (P < 0.001), lower FT3 (P < 0.01) and lower FT3/FT4 (P < 0.001) levels compared to the controls. The HTCL-T-4 group had FT4, FT3 and FT3/FT4 levels equivalent to those of the controls. The HT-alone group had significantly lower FT4 (P < 0.01), equivalent FT3 (P=0.083), and significantly higher FT3/FT4 (P < 0.001) ratios than the controls. Conclusions: The presence of the remnant thyroid tissue was associated with different thyroid hormone balances in thyroidectomized patients, suggesting that T-3 production by remnant thyroid tissue has a substantial effect on the maintenance of postoperative serum T-3 levels.