Association between thyroid hormone balance and thyroid volume in patients with nontoxic nodular goiter

Abstract

Previous studies have suggested that the serum free triiodothyronine (FT3)/free thyroxine (FT4) ratio reflects thyroid hormone metabolism and may be altered in goitrous thyroid diseases. However, its relationship with thyroid volume (TV) in nontoxic nodular goiter remains unclear. We retrospectively analyzed 3,841 patients with thyroid nodules and 1,123 euthyroid controls. Patients were stratified into four groups according to TV. Serum FT3 levels and FT3/FT4 ratios were compared across groups, and their associations with TV were evaluated. The effects of levothyroxine (L-T4) therapy on thyroid hormone levels were also examined in a subset of patients. Serum FT3 levels and FT3/FT4 ratios tended to increase with increasing TV. In patients with large goiters (TV ≥ 80 mL), FT3 levels and FT3/FT4 ratios were significantly higher than those in controls, despite comparable FT4 levels. Both FT3 levels (r = 0.15, p < 0.001) and FT3/FT4 ratios (r = 0.09, p < 0.001) were positively correlated with TV. L-T4 therapy increased FT4 levels and decreased TSH levels, but did not significantly change FT3 levels. In conclusion, larger thyroid volume is associated with higher FT3 levels and FT3/FT4 ratios in patients with nontoxic nodular goiter, suggesting altered thyroid hormone metabolism even in the absence of overt hyperthyroidism.