Thyroidal production of triiodothyronine (T₃) is absent in patients who have undergone total thyroidectomy. Therefore, relative T₃ deficiency may occur during postoperative levothyroxine (l-T₄) therapy. The objective of this study was to evaluate how the individual serum T₃ level changes between preoperative native thyroid function and postoperative l-T₄ therapy.

We retrospectively studied 135 consecutive patients with papillary thyroid carcinoma, who underwent total thyroidectomy. Serum free T₄ (FT₄), free T₃ (FT₃), and TSH levels measured preoperatively were compared with those levels measured on postoperative l-T₄ therapy.

Serum TSH levels during postoperative l-T₄ therapy were significantly decreased compared with native TSH levels (P<0.001). Serum FT₄ levels were significantly increased (P<0.001). Serum FT₃ levels were significantly decreased (P=0.029). We divided the patients into four groups according to postoperative serum TSH levels: strongly suppressed (less than one-tenth of the lower limit); moderately suppressed (between one-tenth of the lower limit and the lower limit); normal limit; and more than upper limit. Patients with strongly suppressed TSH levels had serum FT₃ levels significantly higher than the native levels (P<0.001). Patients with moderately suppressed TSH levels had serum FT₃ levels equivalent to the native levels (P=0.51), and patients with normal TSH levels had significantly lower serum FT₃ levels (P<0.001).

Serum FT₃ levels during postoperative l-T₄ therapy were equivalent to the preoperative levels in patients with moderately suppressed TSH levels. Our study indicated that a moderately TSH-suppressive dose of l-T₄ is required to achieve the preoperative native serum T₃ levels in postoperative l-T₄ therapy.