Association between serum thyroid hormone balance and thyroid volume in patients treated with levothyroxine monotherapy for hypothyroidism.

Many previous studies including ours have reported that athyreotic patients on levothyroxine (LT ₄ ) have relatively low serum free triiodothyronine (FT ₃ ) levels, whereas patients with large goitrous diseases often have high serum FT ₃ levels. Here we investigated Hashimoto thyroiditis (HT) patients on LT ₄ to study the relationship between thyroid volume (TV) and thyroid hormone status in hypothyroid patients on LT ₄ . We retrospectively studied 408 euthyroid HT patients treated with LT ₄ for hypothyroidism; divided them as per TV and compared serum levels of free thyroxine (FT ₄ ) and FT ₃ and the FT ₃ /FT ₄ ratio in each patient group with those in euthyroid matched control group. We also evaluated the association between serum FT ₃ level and FT ₃ /FT ₄ ratio and TV among HT patients on LT ₄ . In patients with TV <15 mL, serum FT ₃ levels were significantly lower than those in controls. In patients with TV 15-80 mL, serum FT ₃ levels were equivalent to those in controls. In patients with TV ≥80 mL, the serum FT ₃ levels were significantly higher than those in controls. The serum FT ₃ level (r = 0.35, p < 0.01) and FT ₃ /FT ₄ ratio (r = 0.42, p < 0.01) showed a positive correlation with TV. TVs in HT patients on LT ₄ caused differences in serum thyroid hormone balance, as increasing volume increases the serum FT ₃ level and FT ₃ /FT ₄ ratio. Serum thyroid hormone balance in HT patients with smaller thyroids was similar to that in athyreotic patients. Mild thyrotropin suppression with LT ₄ is needed to achieve normal FT ₃ levels in such patients.