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.