TSH: what it is, what it tells you and why we test it

TSH is the hormone your pituitary gland, at the base of your brain, uses to tell your thyroid to work harder or less. It works like a seesaw: when your thyroid works too little, TSH rises; when it works too hard, TSH falls. It is the first test for checking your thyroid.

Why we test it

Your thyroid is a small gland at the base of your neck. Its hormones set how fast your body works: heartbeat, temperature, energy, weight.

TSH moves before the thyroid hormones do. That is why it is the first test for your thyroid, and it often spots a problem before any symptoms.

Thyroid problems are common, especially in women, and they often come on slowly. A blood test can find them early.

What it tells you, and what it does not

TSH works the other way round: if it is high, your thyroid is usually working too little. If it is low, it is usually working too hard.

On its own, though, it does not make a diagnosis. Your doctor can ask for FT4, the free thyroid hormone in your blood, and sometimes for thyroid antibodies.

And if your TSH is normal but tiredness or weight gain will not shift, the cause is usually something else: talk to your doctor.

In the Rosa panel, these tests help read it:

Normal TSH levels

The lab decides what is normal. It is usually 0.4 to 4 mIU/L. Your own report may print slightly different limits.

Some reports write the unit as µIU/mL: it is the same unit, and the numbers do not change.

The limit is the same for women and men. With age TSH tends to rise a little, and in pregnancy the limits change.

In the Rosa report you see your TSH next to the rest of the panel, read together with your other tests.

What your result means

Find the row for your result, in mIU/L. TSH alone does not make a diagnosis: your doctor reads it with your symptoms and, if needed, with FT4, which they can ask for.

If your TSH is high or low

What causes high TSH

If TSH is high but FT4 is normal, it is called subclinical hypothyroidism: your thyroid is struggling a little but still coping. Often no treatment is needed, and your doctor checks it again over time. If you are pregnant or trying for a baby, it is different.

Symptoms of high and low TSH

Often none, especially when the value is only slightly out of range. When there are symptoms, they come on slowly:

These symptoms are common and have many other causes. TSH helps show whether your thyroid is involved.

Low TSH: what it can mean

It usually means there is a lot of thyroid hormone in your blood, so the pituitary turns its signal down. In people who take thyroid hormone, a dose that is a little high is often the reason. In others, the thyroid may be working too hard: this is called hyperthyroidism.

But it is not always a problem. In early pregnancy it normally drops, and biotin can make it read lower than it is. If you take thyroid hormone, do not change the dose on your own.

Your doctor usually asks for FT4 and, if needed, other tests. If TSH is only slightly low and FT4 is normal, your doctor often checks it again before deciding.

What can skew the result

A few things change the number for a while. It helps to know them before you read it.

When to worry

Talk to your doctor if:

Frequently asked questions

What is TSH? Thyroid-stimulating hormone, also called thyrotropin: the hormone that stimulates your thyroid. It is made by the pituitary, a small gland at the base of your brain.

Does the normal range for TSH change with age? The limit on your report usually does not. But TSH tends to rise a little with age, even with a healthy thyroid. In an older person a value just above the limit can be normal: your doctor weighs it.

TSH in pregnancy: what changes? In pregnancy the thyroid has to work harder and the limits are different. In the first months TSH usually drops. If you are pregnant or trying for a baby, show your result to your gynaecologist or obstetrician.

Can biotin skew TSH? Yes. High-dose biotin, the kind in hair, skin and nail supplements, can make it read lower than it is. If you take it, ask your doctor whether to stop it at least two days before the blood test, and tell the lab.

How often should it be checked? For people who feel well there is no fixed rule: Rosa measures it once a year. If it is slightly high, your doctor usually repeats it after two or three months, together with FT4.

Sources