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