The blood test for TSH is used to check the level of TSH in the blood, a chemical that your pituitary gland releases to stimulate your thyroid. It's the most useful screen that you could have for a thyroid problem, since the TSH levels change before the thyroid hormones. A high result typically is associated with an underactive thyroid. If it's low, it's likely that it's overactive. There is no required fasting for the test, only one blood tube is required, and your clinician will know if the test was negative or positive, which indicates that further investigation should be done.

What TSH actually is

TSH is not a thyroid hormone. It comes from the pituitary, a pea-sized gland at the base of the brain that works as the thyroid's thermostat. When thyroid hormone runs low, the pituitary releases more TSH to push the thyroid harder. When it runs high, the pituitary backs off and TSH falls. So the number on your report shows how hard your pituitary is working, which is why a TSH test catches problems early.

It is also why the result reads backwards. High TSH, low thyroid. Low TSH, high thyroid.

Who should get a TSH blood test

A TSH blood test is worth ordering if you have been tired for weeks without a clear reason, your weight has moved without a change in habits, or you feel cold when others do not. Dry skin, thinning hair, constipation, and a flat mood point the same way. Racing heart, tremor, anxiety, poor sleep, and weight loss without trying point the other way.

Thyroid problems are several times more common in women, and they interfere with periods, ovulation, and pregnancy. If you are trying to conceive, the NIDDK notes that untreated hypothyroidism raises miscarriage risk. If you are in your forties or fifties and your symptoms look like perimenopause, a TSH test belongs on the same panel as FSH and estradiol, because the two are easy to confuse.

How to prepare

You do not need to fast. Eating has no meaningful effect on TSH.

Timing matters more. TSH runs a little higher in the early morning, and MedlinePlus advises testing early in the day and at the same time each visit so results can be compared.

The one thing that can wreck a result is biotin. High-dose biotin, the vitamin in most hair and nail supplements, interferes with the lab method and can make TSH read falsely low. The FDA has warned about this since 2017. Stop it for at least two days before your draw. If you take levothyroxine, ask whether to take it before or after the draw; the effect is small, but consistency helps.

Reading your result

Each laboratory will have its own reference range, and this is the one that should be followed. Many choose to use values from 0.4 to 4.5 mIU/L, but it will depend on the testers and the equipment used in the lab. A value of 4.3 may be "flagged" at one lab and not another.

High TSH means your thyroid is not working as it should. The next question is "How much" and free T4 answers that. Low T4 and high TSH is an overt hypothyroidism and typically this is treated with levothyroxine. A normal T4 level combined with high TSH levels indicates subclinical, which can be monitored or treated based on TSH, symptoms and plans for pregnancy. Hashimoto's thyroiditis, an autoimmune condition, is the most common cause in the US.

When there is a low level of TSH, the thyroid is overactive, or you are taking too much thyroid medicine. If free T4 or free T3 is elevated and TSH is depressed, it is a manifestation of overt hyperthyroidism, typically due to Graves' disease, nodules or a transient thyroiditis. Subclinical: low TSH, normal hormones, and is usually monitored.

A normal TSH does not rule out thyroid disease, however, if you have no symptoms despite a normal TSH, it is not likely. There can be more than one reason for a normal number to be normal, such as early disease, a pituitary problem, or biotin. Rerepeat and/or have a complete panel is appropriate in a few months.

The trend is to be read with a blood test TSH result. Three TSHs that have risen from 1.5 to 4.0 give you more information than one TSH. There is a detailed explanation of the significance of each band in our guide to the TSH normal range.

What a TSH blood test cannot tell you

TSH says whether something is wrong, not what. A blood test TSH value alone cannot tell Hashimoto's from an iodine problem, or Graves' disease from a nodule.

That is what the rest of a thyroid panel is for. Free T4 and free T3 measure the hormones themselves. Thyroid peroxidase antibodies identify autoimmune thyroiditis, and positive antibodies with a normal TSH predict a higher chance of hypothyroidism later, which is worth knowing before pregnancy. TRAb or TSI antibodies confirm Graves' disease when TSH is low. An American Thyroid Association-commissioned review covers how these fit together, and our full thyroid panel guide explains who needs which.

How to get tested

You can order one yourself. August's lab tests include TSH alone, TSH with free T4, and a full thyroid panel, at one price that covers the draw. A phlebotomist comes to your home for a morning draw, or you visit a nearby lab. Results post the next day in most cases, and a clinician reviews every one before you see it, so an abnormal TSH comes with context rather than a bare number. August Care members get up to 50% off next-day lab tests and a first consultation included, which makes the six-monthly rhythm of thyroid monitoring easier to keep.

Not sure your symptoms point to the thyroid? Describe them to August, a free AI health assistant, to help decide what to test. It is a starting point, not a diagnosis.

Frequently Asked Questions

It measures thyroid-stimulating hormone, which the pituitary gland releases to control the thyroid. The level reflects how hard the pituitary is working to keep thyroid hormone in range. High TSH points to an underactive thyroid, low TSH to an overactive one, and the test is the standard first step in checking thyroid function.

Most labs use a range close to 0.4 to 4.5 mIU/L, but the exact numbers vary by lab and are lower during pregnancy. Use the range printed on your own report rather than one from a website. People on thyroid treatment are often kept in a narrower target range set by their clinician.

A high TSH means the thyroid is not producing enough hormone and the pituitary is compensating. Whether it needs treatment depends on free T4, how high the TSH is, your symptoms, and whether you plan to become pregnant. Hashimoto's thyroiditis, an autoimmune condition, is the most common cause in the US.

A low TSH means the thyroid is making too much hormone, or that thyroid medication is dosed too high. Free T4 and T3 confirm how much. Graves' disease, thyroid nodules, and temporary thyroiditis are the usual causes. Early pregnancy and high-dose biotin can also push TSH low without a thyroid problem.

No. Food does not affect TSH, so you can eat normally. Test in the morning if you can, keep the time consistent between tests, and stop high-dose biotin supplements for at least two days beforehand, since biotin can make TSH read falsely low on many lab methods.

Six to eight weeks after any dose change, because that is how long it takes TSH to settle at the new dose. Once stable, every six to twelve months is typical. Pregnancy, new medications, and a return of symptoms are all reasons to test sooner.

Occasionally. Early disease, a pituitary problem, or biotin interference can hide behind a normal TSH. If symptoms persist, a repeat test in a few months or a fuller panel with free T4, free T3, and antibodies can help your clinician decide.

Yes. August sends a phlebotomist to draw the sample at home, and a certified lab runs the same test a clinic would. Results post the next day in most cases, and a clinician reviews every result. No fasting is needed, so a morning draw before breakfast is simple.