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

Abstract

Many previous studies including ours have reported that athyreotic patients on levothyroxine (LT₄) have relatively low serum free triiodothyronine (FT₃) levels, whereas patients with large goitrous diseases often have high serum FT₃ levels. Here we investigated Hashimoto thyroiditis (HT) patients on LT₄ to study the relationship between thyroid volume (TV) and thyroid hormone status in hypothyroid patients on LT₄. We retrospectively studied 408 euthyroid HT patients treated with LT₄ for hypothyroidism; divided them as per TV and compared serum levels of free thyroxine (FT₄) and FT₃ and the FT₃/FT₄ ratio in each patient group with those in euthyroid matched control group. We also evaluated the association between serum FT₃ level and FT₃/FT₄ ratio and TV among HT patients on LT₄. In patients with TV <15 mL, serum FT₃ levels were significantly lower than those in controls. In patients with TV 15–80 mL, serum FT₃ levels were equivalent to those in controls. In patients with TV ≥80 mL, the serum FT₃ levels were significantly higher than those in controls. The serum FT₃ level (r = 0.35, p < 0.01) and FT₃/FT₄ ratio (r = 0.42, p < 0.01) showed a positive correlation with TV. TVs in HT patients on LT₄ caused differences in serum thyroid hormone balance, as increasing volume increases the serum FT₃ level and FT₃/FT₄ ratio. Serum thyroid hormone balance in HT patients with smaller thyroids was similar to that in athyreotic patients. Mild thyrotropin suppression with LT₄ is needed to achieve normal FT₃ levels in such patients.