Background <p>We have demonstrated that mild suppression of thyrotropin (TSH) using levothyroxine (LT₄) is necessary to achieve normal free triiodothyronine (FT₃) levels and a metabolic euthyroid state in athyreotic patients. Consequently, management strategies based on thyroid volume (TV) have been implemented in patients receiving LT₄ at Kuma Hospital. While Hashimoto’s thyroiditis (HT) can vary from atrophic to diffusely enlarged thyroid volumes.</p> Methods <p>In this retrospective study, we aimed to evaluate the composition of thyroid hormone measurement items (serum FT₄, FT₃, and FT₄+FT₃) in patients receiving LT₄ monotherapy for HT, stratifying them by TV, as assessed using ultrasonography.</p> Results <p>We identified 7,576 consecutive patients from their medical records. These patients were classified into five groups based on TV: atrophic thyroid (TV ≤ 10 mL); normal thyroid (TV 10–20 mL); mildly enlarged thyroid (TV 20–50 mL); moderately enlarged thyroid (TV 50–80 mL); and large thyroid (TV &gt; 80 mL). In all patient groups, FT₄ measurements constituted the majority or nearly half of all assessments. However, the proportion of FT₃ measurements increased notably in the atrophic thyroid group (34%) and in the large goiter group (29%). Patients receiving LT₄ were evaluated based on their TSH levels. In the reduced TSH group, FT₃ accounted for nearly half of all measurements (47%). Conversely, in the normal TSH group, FT₄ accounted for the majority of all measurements (76%).</p> Conclusion <p>This study revealed that FT₃, rather than FT₄, was the predominant thyroid hormone measured in a significant number of patients with HT on LT₄ monotherapy at our hospital. Furthermore, our study highlights a management approach whereby TSH levels were intentionally suppressed in some patients.</p>