Overview
What Levothyroxine Replaces
Levothyroxine is synthetic thyroxine (T4), the principal hormone produced by the thyroid gland.
Levothyroxine is synthetic thyroxine (T4), the principal hormone produced by the thyroid gland. Peripheral tissues convert T4 to the more active hormone, T3. Replacement restores the metabolic signalling that is inadequate in hypothyroidism. In primary hypothyroidism, the thyroid produces too little T4 and T3. The pituitary senses the low circulating hormone level and responds by increasing thyroid-stimulating hormone (TSH). TSH is therefore both a diagnostic clue and the main laboratory marker used to guide replacement therapy. Levothyroxine works slowly because it replaces a hormone with a long physiologic effect rather than treating symptoms immediately. Fatigue, constipation, dry skin, and slowed thinking may improve gradually. TSH also takes time to equilibrate after a dose change, which is why a dose should not be increased simply because the patient still feels tired a few days after starting treatment. Oral levothyroxine is first-line therapy for overt hypothyroidism and for most patients with subclinical hypothyroidism selected for treatment.