Which thyroid hormone should not be given alone?

Liothyronine, a synthetic form of the active thyroid hormone T3, should not be given alone in most cases because it can lead to significant fluctuations in thyroid hormone levels. This can result in a rapid increase in metabolic activity, potentially causing symptoms of hyperthyroidism, such as increased heart rate, anxiety, and weight loss.

In contrast, levothyroxine (T4) is typically given alone as it provides a more stable and manageable way to maintain thyroid hormone levels. It is converted to T3 in the body as needed. Treatments with desiccated thyroid or thyroid extract also contain T4 and T3 in a natural proportion and can be used, but they are usually not the first-line choice. Therefore, the consideration of T3 therapy often requires the addition of T4 to maintain a balanced hormonal output and avoid the adverse effects associated with excess T3.