Proportion of serum thyroid hormone concentrations within the reference ranges in athyreotic patients on levothyroxine monotherapy: a retrospective study

BACKGROUND: In patients receiving thyroid-stimulating hormone (TSH) suppressive therapy with levothyroxine (LT(4)) after total thyroidectomy for thyroid cancer, thyroid function tests should be performed to adjust the LT(4) 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 (FT(4)) or free triiodothyronine (FT(3)) indicates better for adjusting the dose in athyreotic patients on LT(4) monotherapy after total thyroidectomy. METHODS: We retrospectively studied the compatibility of free thyroid hormone (FT(4) and FT(3)) concentrations with reference ranges in athyreotic patients on LT(4) monotherapy after total thyroidectomy. RESULTS: We identified 2210 consecutive patients from their medical records. Of these patients, 250 had both FT(4) and FT(3) 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 FT(4) concentrations within the reference range (0.9–1.7 ng/dL) and 146 patients (70.5%) had FT(4) concentrations above the reference range. In contrast, 10 patients (4.8%) had FT(3) concentrations below the reference range (2.3–4.0 pg/mL) and 8 (3.9%) had FT(3) 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 FT(4) concentrations within the reference range and 18 (41.9%) had FT(4) concentrations above the reference range. While, 11 patients (25.6%) had FT(3) concentrations below the reference range and one (2.3%) had FT(3) concentrations above the reference range; hence, 31 patients (72.1%) had FT(3) concentrations within the reference range. CONCLUSION: This study showed that measuring FT(3) concentrations rather than FT(4) 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, FT(3) measurement could be useful in providing more detailed treatments, including avoiding more aggressive TSH suppressive therapy and identifying the presence of low T(3) syndrome in the background.