Liothyronine (T3)

Status

FDA-approved. FDA-approved labeling exists as Cytomel for hypothyroidism, myxoedema, and as a diagnostic agent in thyroid suppression tests. It carries the thyroid hormone class boxed warning against use for obesity or weight reduction, noting that larger doses may produce serious or life-threatening toxicity.

What it is

An active thyroid hormone prescribed for hypothyroidism.

What published research measured

Synthetic liothyronine, the active thyroid hormone, which in normal physiology is mostly produced by peripheral conversion from thyroxine rather than secreted directly. Its pharmacokinetics are the central issue: a short half-life produces peaks and troughs that thyroxine's long half-life smooths out, making stable dosing harder and increasing the scope for transient thyrotoxic symptoms. Randomized trials comparing combination thyroxine-plus-liothyronine against thyroxine alone have largely failed to show consistent superiority, though a subgroup of patients reports preference for combination therapy and the question remains actively researched. The cardiac risk of over-replacement — atrial fibrillation in particular — arrives faster with liothyronine than with thyroxine. The same boxed prohibition on use for weight loss applies.

Source

FDA labeling for Cytomel, including the boxed warning against use for weight loss

Sources

• Half-life source for Liothyronine (T3)