Thyroid test results: TSH and free T4

The two main tests

• TSH comes from the pituitary gland and tells the thyroid how much hormone to make. When thyroid hormone is low, TSH goes up; when it is high, TSH goes down. That is why a high TSH most often means an underactive thyroid and a low TSH usually means an overactive one. (MedlinePlus, 2024) (NIDDK)
• Free T4 is the part of the main thyroid hormone that is not bound to proteins and can act on the body. Free T4 (or a free T4 index) reflects thyroid function more accurately than total T4 when read with TSH. (ATA) (MedlinePlus T4, 2024)
• T3 and thyroid antibody tests may follow to find the cause, for example Hashimoto's disease or Graves' disease. (MedlinePlus, 2024) (ATA)

Common result patterns

"Overt" means TSH and T4 are both outside the range. The USPSTF uses "thyroid disease" for overt changes that persist and come with clear symptoms that nothing else explains better. (USPSTF, 2015)

Why lab ranges differ

Each lab sets its range from its own test, so compare your result with the range printed on your report. Most labs use about 0.4 to 4.5 mIU/L for TSH, but there is no consensus on the cut points. TSH also varies by age, sex and race or ethnicity. (USPSTF, 2015)

Bodies differ on what counts as subclinical. The USPSTF describes the upper cut point of 4.5 mIU/L as common but arbitrary. Subclinical hypothyroidism is often split into TSH 4.5 to 10.0 and above 10.0 mIU/L. Subclinical hyperthyroidism is often split into about 0.1 to 0.4 and below 0.1 mIU/L. (USPSTF, 2015)

Why TSH is often repeated

The USPSTF says multiple TSH tests over 3 to 6 months should be done to confirm or rule out an abnormal result. Free T4 is then checked if TSH stays abnormal, to tell subclinical from overt. (USPSTF, 2015)

Where bodies differ: as the USPSTF summarizes them, many professional groups advise repeating over 3 to 6 months before a diagnosis in people without symptoms, unless TSH is above 10.0 or below 0.1 mIU/L. (USPSTF, 2015) Mild changes often settle: about 37% of people with subclinical hypothyroidism return to normal on their own after several years, and about one quarter of people with subclinical hyperthyroidism do. (USPSTF, 2015)

What can skew a result

• Illness: some serious illnesses not related to the thyroid can lower TSH for a short time. (MedlinePlus, 2024)
• Age: TSH may be higher in people over 80 without a thyroid problem; 12% of people aged 80 or older with no thyroid disease had a TSH above 4.5 mIU/L in data the USPSTF cites. (MedlinePlus, 2024) (USPSTF, 2015)
• Pregnancy: TSH is often a little low in the first three months. Pregnancy also raises the proteins that bind thyroid hormone, which raises total T4, so TSH with free T4 is the better check. (MedlinePlus, 2024) (ATA)
• Biotin: this common over-the-counter supplement can make several thyroid tests look abnormal when they are normal. The ATA advises not taking it for 2 days before the blood draw. (ATA)
• Medicines: some medicines change T4 levels. Tell whoever orders the test everything you take, and do not stop a medicine unless your doctor tells you to. (MedlinePlus T4, 2024) (MedlinePlus, 2024)

An abnormal T4 does not always mean a condition that needs treatment. Your doctor reads the result with your symptoms, history and other tests. (MedlinePlus T4, 2024)

Screening vs testing for a reason

For adults without symptoms who are not pregnant, the USPSTF found the evidence insufficient to recommend for or against routine thyroid screening (I statement). As the USPSTF summarizes them, the American Thyroid Association and the American Association of Clinical Endocrinologists suggest considering screening after 60 and looking actively in people at higher risk and women planning pregnancy. (USPSTF, 2015) See thyroid test: who needs one.

Questions people ask

What is a normal TSH level?

Each lab sets its own range from its own test, so read the range printed on your report. The USPSTF notes that most labs use about 0.4 to 4.5 mIU/L, and that there is no agreement on the exact cut points. (USPSTF, 2015)

What does a high TSH with a normal free T4 mean?

This pattern is called subclinical hypothyroidism: TSH is above the lab range while T4 is normal. The USPSTF reports that about 5% of women and 3% of men in the United States have it, and that about 37% return to normal on their own after several years. Your doctor decides what it means for you. (USPSTF, 2015)

Should an abnormal TSH be repeated?

Usually yes when you have no symptoms. The USPSTF says multiple tests over 3 to 6 months should confirm or rule out an abnormal result, and notes that many professional groups advise repeating before a diagnosis unless TSH is above 10.0 or below 0.1 mIU/L. (USPSTF, 2015)

Can a supplement change thyroid test results?

Yes. The American Thyroid Association says biotin, a common over-the-counter supplement, can make several thyroid tests look abnormal when they are normal, and advises not taking it for 2 days before the blood draw. Tell whoever orders the test everything you take, and do not stop a medicine unless your doctor says so. (ATA) (MedlinePlus, 2024)

Is TSH different in older adults?

Yes. TSH tends to run higher with age: the USPSTF cites data that 12% of people aged 80 or older with no thyroid disease have a TSH above 4.5 mIU/L, and says the standard range is probably not right for older adults. MedlinePlus also notes TSH may be higher after age 80 without a thyroid problem. (USPSTF, 2015) (MedlinePlus, 2024)

Related

• Thyroid test: who needs one
• Thyroid cancer screening: neck checks, scans and nodules
• Blood tests by age
• Annual physical tests by age
• USPSTF grades explained
• Tests you may not need