The association between thyroid hormone balance and thyroid volume in patients with Hashimoto thyroiditis

While patients with large goitrous thyroid diseases often have a relatively high serum free triiodothyronine (FT₃)/free thyroxine (FT₄) ratio, athyreotic patients have a relatively low FT₃/FT₄ ratio. Here we investigated the relationship between thyroid hormone status and thyroid volume (TV) among a large number of euthyroid Hashimoto thyroiditis (HT) patients. We retrospectively enrolled 2,603 untreated HT patients who visited the Kuma hospital from 2012 to 2016, and divided them into four groups as per the TV: normal TV (<20 mL), slight goiter (20 ≤ TV < 50 mL), moderate goiter (50 ≤ TV < 80 mL), and the large goiter group (≥80 mL). Baseline characteristics and laboratory data of each group were compared to those of 1,554 control subjects. The association between FT₃/FT₄ ratio and TV among HT patients was then analyzed. We observed a change in laboratory parameters among 13 patients in the large goiter group who were prescribed levothyroxine (LT₄) for reducing TV. Compared to normal subjects, the moderate and large goiter groups exhibited significantly higher serum FT₃ levels, while all HT groups exhibited lower serum FT₄ levels. Serum FT₃/FT₄ ratios showed a positive correlation with TV (r = 0.35, p < 0.01), which was independent of age, sex, body mass index, and TgAb and TSH levels. LT₄ treatment lowered serum FT₃ levels and FT₃/FT₄ ratios significantly. Our results indicated that HT patients with increased TV tended to present with high serum FT₃, low FT₄, and high FT₃/FT₄ ratios. The elevation of deiodinase activity may be an important factor affecting thyroid hormonal balance in such patients.