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

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.