Comparison of Thyroid Hormones in Euthyroid Athyreotic Patients Treated with Levothyroxine and Euthyroid Healthy Subjects

ABSTRACT

BACKGROUND AND OBJECTIVES: Levothyroxine (L-T4) monotherapy to normalize TSH level might be not sufficient to restore serum free triiodothyronine (fT3) levels in hypothyroid patients. This study aimed to compare the thyroid hormone levels in euthyroid L-T4 treated athyreotic patients and euthyroid healthy control subjects. MATERIALS AND METHODS: We included 69 euthyroid L-T4 treated athyreotic female patients after total thyroidectomy and radioactive iodine ablation therapy and 90 euthyroid healthy female. Serum fT3 and free thyroxine (fT4) levels were simultaneously measured using two different assay kits (A and B). RESULTS: The serum fT4 level was higher in the athyreotic patients (kit A p<0.001, kit B p=0.046), and the serum fT3 level was higher in control subjects (kit A p=0.047, kit B p=0.102). In the control group, the serum fT3 level was stable and not correlated with the TSH level (kit A tau=−0.10, p=0.18, kit B tau=−0.06, p=0.40). However, in the patient group, the serum fT3 level was negatively correlated with the TSH level (kit A tau=−0.22, p=0.012, kit B tau=−0.31, p<0.001). All thyroid hormone parameters measured by kit A showed higher area under the curve values than those measured by kit B for distinguishing the patients from the control subjects. CONCLUSION: The serum fT3 levels in L-T4 treated athyreotic euthyroid patients were significantly lower than and varied from those of euthyroid healthy control subjects. Thus, L-T4 monotherapy might not be appropriate for some athyreotic patients to maintain optimal T3 levels.