Current status of the thyroid hormone measurement items in patients receiving levothyroxine monotherapy by the management based on the thyroid tissue volume

We and other investigators reported that mild TSH suppression with levothyroxine (LT₄) was needed to achieve normal free triiodothyronine (FT₃) levels and metabolic euthyroid state in athyreotic patients. Consequently, management methods based on thyroid tissue volume have been implemented for patients receiving LT₄ at the Kuma Hospital. This retrospective study examined the composition of the thyroid hormone measurement items (serum-free thyroxine [FT₄], FT₃, and FT₄ + FT₃) in patients receiving LT₄ monotherapy. According to the etiology of hypothyroidism, 36% of the 25,523 patients included in this study underwent total thyroidectomy (TT). Thirteen percent and 14% had undergone ¹³¹I treatment for hyperthyroidism (RIT) and partial thyroidectomy (PT), respectively. Moreover, 37% of patients had received non-invasive treatment (NIT). The proportion of patients who underwent only FT₃ measurements was higher (TT, 93%; RIT, 61%) in the first two groups, whereas the proportion of patients who underwent only FT₄ measurements was higher (PT, 50%; NIT, 65%) in the remaining two groups. Only FT₃ measurements were performed in 58% of patients. Only FT₄ measurements were performed in 34% of patients. The serum TSH levels were suppressed in nearly half of the patients (46%). Thus, FT₃ was the major thyroid hormone measured in patients receiving LT₄ treatment, and the serum TSH levels were suppressed in nearly half of the patients. This may be attributed to the management guidelines at our hospital, a specialized facility for thyroid disease, wherein half of the patients present are athyreotic or have atrophic thyroid glands after TT or RIT.