What Does a TSH Number Mean?

You have received your thyroid test result and see a number next to TSH, and maybe a flag or an asterisk. What does this number say, and what does it not say?

What is TSH?

MedlinePlus says TSH is a hormone made by the pituitary gland to tell the thyroid to make its hormones. So TSH is not itself a thyroid hormone. It is the pituitary’s message to the thyroid. When the thyroid is underactive, the pituitary shouts louder and TSH goes up. When the thyroid is overactive, the pituitary goes quiet and TSH comes down.

The NHS laboratory in Gloucestershire calls TSH the first test for thyroid disease. Depending on the result or the circumstances, the lab also adds FT4 and sometimes FT3. These two measure the free hormone circulating in the blood.

What does high mean? What does low mean?

• High TSH usually points to an underactive thyroid, which has many causes, such as Hashimoto’s (MedlinePlus). The NHS laboratory says a high TSH with a low FT4 is consistent with an underactive thyroid.
• Low TSH usually points to an overactive thyroid. Causes: Graves’ disease, active nodules, too much iodine (for example CT contrast material), or extra thyroid hormone from a medicine or supplement (MedlinePlus). The same source says some medicines, such as corticosteroids and morphine, lower TSH.

A single number outside the range is not a diagnosis by itself. NIDDK says symptoms cannot make a definite diagnosis, and the doctor puts together blood tests and sometimes imaging. Likewise, a number within the range does not always mean your symptoms have nothing to do with the thyroid. That judgment is up to the doctor.

What does “normal” mean?

The normal range is not fixed.

• The NHS laboratory in Gloucestershire lists the adult TSH range as 0.27 to 4.2.
• MedlinePlus lists the usual range as 0.4 to 4.8 and says experts do not agree on the upper limit, and some labs set a higher limit for older adults. Compare your number with the range on that same report.
• In pregnancy the range differs in each trimester. The NHS laboratory lists a separate range for the first, second and third trimester.
• For children the range is based on age.

MedlinePlus says even for someone being treated, the target is usually this same range, but there are exceptions: when the cause of the underactive thyroid is the pituitary, a low TSH is expected, after some thyroid cancers the TSH may be kept below the usual range, and in pregnancy it has its own range. So your own target is set by your doctor.

What can change the number?

• Time of day: MedlinePlus says TSH changes during the day, and early morning is the best time.
• Illness: The NHS laboratory says an acute illness can make the result misleading, and the test is better done about three months later.
• Medicines: Amiodarone and lithium affect the thyroid. MedlinePlus says to tell the doctor about all your medicines, but not to stop any of them without asking.
• Biotin: Both sources, the NHS and MedlinePlus, say biotin distorts the result. If you take a hair and nail supplement, say so before the test.
• Pituitary disease: The NHS laboratory says that in this case FT4 should be measured along with TSH, because TSH alone is misleading.
• Overactive thyroid under treatment: The same laboratory says TSH may stay low for months, so in following an overactive thyroid, FT4 and FT3 give better information.

After starting a thyroid pill

NIDDK says the blood test is usually repeated six to eight weeks after starting the pill, and the dose is adjusted if needed. After every change, the test is done again. Once the dose has settled, testing is at first about every six months and then once a year. The NHS laboratory lists waiting at least two months after a change in treatment. Testing too early gives a misleading result. The doctor sets the interval.

The NHS says to take the pill on an empty stomach, 30 to 60 minutes before the first meal, and at the same time every day. For details, see the page “Underactive and Overactive Thyroid”.

What to ask

• What is the normal range for me?
• How is my number different from last time, and were the test conditions (time, pill, biotin) the same?
• Do I also need FT4 or another test?
• When should I be tested again?
• What sign should make me call sooner?

Raise, lower or stop your thyroid pill only on the doctor’s advice. A number close to the range or slightly outside it is not a reason to change the medicine without the doctor. If you have a severe and sudden symptom, see the guide “When should I call right away?”.