Levothyroxine (T4)

Status

FDA-approved. FDA-approved labeling exists for hypothyroidism as replacement therapy and for pituitary TSH suppression in specific thyroid cancer contexts. It carries a boxed warning stating that thyroid hormones must not be used for the treatment of obesity or weight loss, and that larger doses may produce serious or life-threatening toxic effects, particularly when combined with sympathomimetic amines.

What it is

The standard thyroid replacement hormone, taken daily.

What published research measured

Synthetic thyroxine, a narrow therapeutic index medicine and one of the most prescribed drugs in the world, with unambiguous evidence for hypothyroidism replacement. The clinically important material is in the details of use rather than in whether it works. Absorption is substantially reduced by food, coffee, calcium, iron, proton pump inhibitors and bile acid sequestrants, which is why consistent administration conditions matter as much as the amount. Steady state takes around six weeks, so titration on earlier testing chases noise. Over-replacement is not benign — suppressed TSH is associated with atrial fibrillation and reduced bone mineral density, particularly in older adults — and treating a normal thyroid for weight loss or energy carries those risks without the corresponding benefit.

Source

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

Sources

• Half-life source for Levothyroxine (T4)