Liothyronine

Liothyronine is a synthetic form of the thyroid hormone triiodothyronine (T3). It is used as a replacement hormone in hypothyroidism and, in some settings, as part of diagnostic testing or acute treatment in hospital. Liothyronine is more potent and has a shorter duration of action than the more commonly used levothyroxine (T4).

The medication is available in oral tablet form and as an injectable intravenously labeled product for hospital use. When taken by mouth, liothyronine is absorbed rapidly, with a relatively short half-life compared with T4, which can lead to quicker onset of action and a need for more frequent dosing. Because of its potency and faster pharmacokinetics, liothyronine can increase the risk of cardiac side effects and is typically used under careful supervision, especially in older patients or those with cardiovascular disease.

Common indications include replacement therapy for hypothyroidism, often in patients who have not achieved symptom control with levothyroxine alone or in specific clinical circumstances where rapid thyroid hormone action is desired. In practice, liothyronine is not usually the first-line monotherapy for chronic hypothyroidism; levothyroxine remains the standard treatment, with liothyronine used selectively and under specialist guidance.

Contraindications include untreated thyrotoxicosis and certain forms of heart disease. Cautions focus on the risk of hyperthyroid symptoms, interactions with other medications (such as iron, calcium, and certain acid-reducing agents), and the need to treat potential adrenal insufficiency before initiating thyroid hormone therapy. Adverse effects resemble hyperthyroidism and can include palpitations, anxiety, weight loss, and insomnia.