Thyroid hormone measurement items in patients with Hashimoto's thyroiditis on levothyroxine monotherapy: management...

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.

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.

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 > 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%).

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.