Levothyroxine

What it is

Levothyroxine is the synthetic form of T4, the main thyroid hormone. An underactive thyroid is common, especially in women, and causes fatigue, weight gain, cold intolerance and low mood that resolve with the right dose.

The right dose is found with blood tests (TSH) every six to eight weeks until stable, then checked yearly. It is one of the most prescribed medicines in the United States.

FDA-approved uses

• Hypothyroidism of any cause
• Pituitary TSH suppression in some thyroid cancers

Our doctors prescribe for approved uses. Any other use is a decision made only by your doctor, for you, after evaluation.

How it is taken

One tablet every morning on an empty stomach, 30 to 60 minutes before food, coffee or other medicines. Calcium and iron are kept at least four hours apart.

What to expect

Symptoms improve over several weeks once the dose is right. Too much causes palpitations, anxiety, sleep trouble and weight loss — which is why it is never used as a weight-loss medicine.

Who should not take it

• Weight loss in people with normal thyroid function — this is dangerous and not permitted
• Untreated adrenal insufficiency
• Recent heart attack without specialist input

Questions we hear