Thyroid function tests: what TSH and free T4 show, and when treatment is offered

TSH runs backwards, and that is the whole trick

On thyroid function tests, read TSH first, and remember that it moves the opposite way to the gland: a high TSH suggests an underactive thyroid, a low TSH an overactive one. TSH is not a thyroid hormone at all but the pituitary’s signal telling the thyroid to work harder, and a 2024 endocrinology review calls it the single best first test for thyroid disease.

The pituitary behaves like a thermostat. When it senses too little thyroid hormone it turns the signal up, and when it senses too much it turns it down. That is why the number on your results moves in the opposite direction to the gland it describes.

Many UK laboratories run what the British Thyroid Foundation calls a cascade: TSH is measured first, free T4 is added if TSH is above the range, and free T4 and free T3 if it is below. NICE recommends the same approach for adults when a pituitary problem is not suspected. So a sheet showing only TSH is not an incomplete routine panel; it usually means TSH was normal.

Which matters most: TSH, T4 or T3?

TSH answers first, and free T4 says how far things have gone. UK laboratories give TSH in mU/L and free T4 and free T3 in pmol/L. As a guide, the British Thyroid Foundation gives typical adult ranges of 0.4 to 4.0 mU/L for TSH, 9 to 25 pmol/L for free T4 and 3.5 to 7.8 pmol/L for free T3, while stressing that each laboratory sets its own. Read together, TSH and free T4 fall into a handful of patterns.

High TSH, normal T4: what NICE recommends

Subclinical hypothyroidism is far more common than the overt kind, and it is the result people most often bring home with a question mark. NICE does not ask for treatment on a single value. It looks for the same picture on two tests three months apart.

For adults whose TSH is 10 mU/L or higher on both occasions, NICE suggests considering levothyroxine. For adults under 65 whose TSH is above the range but below 10, with symptoms, it suggests considering a six-month trial, and stopping if symptoms persist once TSH is back in range. A thyroid antibody test (TPO antibodies) may be done once to judge the risk of the thyroid slowly failing; NICE advises not repeating it.

That caution reflects the evidence. A 2023 review concludes that this picture is linked with outcomes halfway between normal and overt hypothyroidism, but that evidence that hormone replacement helps is lacking. Treatment is not automatically the answer to a flagged TSH.

Is high or low worse? The low side has its own risk

A TSH below the range is not the harmless end of the scale. The British Thyroid Foundation warns that over-replacement, to the point where TSH becomes undetectable, may cause osteoporosis and long-term harm to the heart and circulation.

That matters for anyone on levothyroxine: NICE asks doctors to keep TSH within the reference range and to avoid doses that suppress it. The aim is a TSH inside the range, not as low as possible.

Why the same person gets different TSH values

TSH follows a marked daily rhythm: values are clearly lower in the afternoon and evening than in the early morning, and that swing within one day is larger than the differences between the analysers of different manufacturers. A sample taken at eight in the morning and one taken at four in the afternoon are not directly comparable. When one laboratory adjusted its TSH limits for the hour of sampling, apparently overactive results fell markedly, above all in afternoon samples.

Medicines move the picture too. The British Thyroid Foundation asks you to tell your doctor about the contraceptive pill, steroids, anti-epileptic and anti-inflammatory medicines, lithium and amiodarone, and any supplement containing iodine or biotin.

Biotin, sold in high doses for hair, skin and nails, deserves its own warning. Many thyroid tests use a biotin-based chemistry, and a high blood level can push free T4 and free T3 falsely up and TSH falsely down: the exact pattern of an overactive thyroid, in someone whose thyroid is fine.

Before you act on a thyroid result

Each reference range leaves about 1 in 20 healthy people outside it, and TSH shifts through the day, so a single mild abnormality is a reason to recheck before deciding anything. A result that does not fit how you feel is also a reason to ask whether something interfered with the test.

If you take levothyroxine, NICE suggests checking TSH every three months until it is stable, then once a year; ask whether to take your dose before or after the blood test, and do it the same way each time. If you are pregnant or planning a pregnancy, make sure whoever reads your results knows, because the TSH range is different in pregnancy.