Thyroid hormoneT3 · T4 · Thyroxine · Triiodothyronine

Thyroid hormones set the metabolic pace of nearly every tissue. The thyroid produces mainly T4, a relatively inactive storage form, which is converted in the tissues into T3, the active hormone that acts on cell nuclei to change gene expression.

The system is controlled by a feedback loop. The pituitary releases TSH to stimulate the thyroid, and rising thyroid hormone suppresses TSH in return, which is why TSH is the first test used and why it moves in the opposite direction to the hormones it controls.

Too little produces hypothyroidism: fatigue, weight gain, feeling cold, dry skin, constipation, low mood, slowed heart rate. Too much produces the reverse: weight loss, heat intolerance, anxiety, tremor, palpitations. Both are common, both are treatable, and both are diagnosed with blood tests rather than symptoms.

The symptom overlap with ordinary life is the reason thyroid testing is ordered so often and the reason a normal result in someone exhausted is useful information rather than a dismissal.

Two things distort measurement and are worth mentioning before a blood test: biotin supplements interfere with many thyroid assays, and acute illness changes results in ways that can mimic disease.

Metabolic rate is genuinely thyroid dependent, which is why the gland gets blamed for weight that has other causes, and why thyroid hormone has a long unhappy history as a weight loss drug. Using it that way is dangerous, affecting heart rhythm and bone, and it is not a legitimate treatment for obesity.

In practice, take symptoms and an abnormal TSH to a GP, and treat thyroid support supplements, which sometimes contain actual hormone, with real suspicion.