Comparison of Triiodothyronine Level in Patients Treated with Levothyroxine for Different Causes of Hypothyroidism

Objective: Several lines of evidence suggest that the symptoms of hypothyroidism, including psychological symptoms and metabolic effects, persist in a significant percentage of patients treated with levothyroxine (L-T4). A hypothesis to explain this phenomenon is that the triiodothyronine (L-T3) serum levels may not be completely normalized. This study aimed to compare the level of serum free T3 (FT3) in patients who are biochemically euthyroid after L-T4 monotherapy with different underlying causes of hypothyroidism. Material and Methods: This cross-sectional study was conducted on patients with hypothyroidism who received L-T4 monotherapy and were biochemically euthyroid. The serum levels of thyroid-stimulating hormone (TSH), free T4 (FT4), FT3, and FT3/FT4 ratio were measured in these patients. Patients were divided into three groups based on the cause of hypothyroidism (radioiodine therapy, thyroidectomy and Hashimoto), and the results of biochemical tests of the thyroid were compared in three groups. Results: Of the 78 patients studied, 12 (15.4%) cases were male, and 66 (84.6%) cases were female. Among the variables studied, only the mean value of FT3 was significantly different in the three groups (p=0.006), where the highest mean value was seen in the Hashimoto group, and the lowest mean value was seen in the radioiodine therapy group. Conclusion: The results of this study showed that although hypothyroidism patients can be optimally treated with L-T4 alone, in many of these patients, the level of FT3 will not be in therapeutic range, and the mean serum FT3 levels in these patients could be related to their hypothyroidism causes.