Proportion of serum thyroid hormone concentrations within the reference ranges in athyreotic patients on...

In patients receiving thyroid-stimulating hormone (TSH) suppressive therapy with levothyroxine (LT₄) after total thyroidectomy for thyroid cancer, thyroid function tests should be performed to adjust the LT₄ dose. Specifically, serum TSH concentrations are commonly measured because TSH suppression is necessary according to thyroid cancer risk. The aim of the present study was to elucidate whether free thyroxine (FT₄) or free triiodothyronine (FT₃) indicates better for adjusting the dose in athyreotic patients on LT₄ monotherapy after total thyroidectomy.

We retrospectively studied the compatibility of free thyroid hormone (FT₄ and FT₃) concentrations with reference ranges in athyreotic patients on LT₄ monotherapy after total thyroidectomy.

We identified 2210 consecutive patients from their medical records. Of these patients, 250 had both FT₄ and FT₃ concentrations in addition to TSH. Two hundred seven had serum TSH concentrations below the reference range (0.5-5.0 μIU/mL), while 43 had them within the reference range. In the 207 patients with TSH concentrations below the reference range, 61 patients (29.5%) had FT₄ concentrations within the reference range (0.9-1.7 ng/dL) and 146 patients (70.5%) had FT₄ concentrations above the reference range. In contrast, 10 patients (4.8%) had FT₃ concentrations below the reference range (2.3-4.0 pg/mL) and 8 (3.9%) had FT₃ concentrations above the reference range; 189 patients (91.3%) had concentrations within the reference range. Of the 43 patients with TSH concentrations within the reference range, 25 (58.1%) had FT₄ concentrations within the reference range and 18 (41.9%) had FT₄ concentrations above the reference range. While, 11 patients (25.6%) had FT₃ concentrations below the reference range and one (2.3%) had FT₃ concentrations above the reference range; hence, 31 patients (72.1%) had FT₃ concentrations within the reference range.

This study showed that measuring FT₃ concentrations rather than FT₄ concentrations as the subsequent parameter of thyroid function might be more useful for disease management in terms of the proportion of serum thyroid hormone concentrations within the reference ranges. Furthermore, FT₃ measurement could be useful in providing more detailed treatments, including avoiding more aggressive TSH suppressive therapy and identifying the presence of low T₃ syndrome in the background.