Levothyroxine Therapy and T₃ Levels

Only 15% to 20% of active triiodothyronine (T₃) is produced by the thyroid gland; the rest is derived from peripheral conversion of thyroxine (T₄). Thus, that combined treatment with T₄ plus T₃ in athyreotic patients (i.e., those who have undergone total thyroidectomy) has not been proven better than T₄ alone is not surprising. This issue was explored further in 50 euthyroid patients who underwent total thyroidectomy: 33 for benign disease and 17 for malignant disease.

Thyroid-stimulating hormone (TSH), free T₄, and total T₃ levels were measured just before surgery and at 2 and 4 months postoperatively. Postoperative T₄ doses were adjusted to achieve low-normal TSH levels in patients with benign disease (initial dose, 1.7 µg/kg) and subnorma…