Thyroid panel

What it tells you

• Small changes in thyroid hormone cause large changes in TSH. That makes TSH more sensitive than the hormones themselves.
• TSH and free T4 together define the conditions. A high TSH with a low free T4 is an underactive thyroid. A high TSH with free T4 still in range is called subclinical hypothyroidism.
• Free T3 has a narrow role, mainly when the thyroid may be overactive. Guidelines say it should not be used to diagnose an underactive thyroid.
• TSH rises with age in healthy people. Many older adults with a TSH above the usual adult range are within the range for their age.
• One mildly raised TSH is often temporary, so it is repeated with free T4, ideally after two to three months.
• Most mildly raised TSH does not need treatment. A 2019 BMJ guideline recommends against thyroid hormone for almost all adults with a raised TSH and normal free T4.

What can throw it off

• Biotin, common in hair, skin and nail supplements, can make TSH read falsely low and free T4 falsely high on some tests. Together this can look like Graves' disease, an overactive thyroid. Stop biotin for a few days before the draw, and tell whoever orders the test.
• TSH follows a daily rhythm, varying by up to about half its average and peaking at night. Repeat tests are best drawn at a similar time of day.
• Recovery from an illness can raise TSH for a while. A serious illness lowers T3 first, then TSH or T4, as the body adapts.
• Some medicines lower TSH, including steroids, dopamine-type drugs, metformin and opioids. Others, such as metoclopramide, lithium and amiodarone, can raise it.
• Estrogen and pregnancy raise the protein that carries thyroid hormone in the blood. Testosterone and steroid medicines lower it. These shifts can mislead hormone results.
• Pregnancy needs its own trimester-specific ranges.
• Interfering antibodies in some people's blood can make TSH read falsely high. Retesting on a different lab method sorts this out.
• TSH is often higher in people with obesity, and on average it runs lower in Black adults than in White adults.

Evidence

Limits

• Normal is a population range, not a health target. The upper limit depends on the reference group, age and lab method.
• A single raised TSH is often temporary. Illness recovery, timing, medicines and lab interference all move it.
• A mildly raised TSH in older adults is often part of healthy aging.
• Trials have not shown benefit from treating a mildly raised TSH.
• TSH can mislead when the pituitary gland itself is the problem. Free T4 is the test then.
• Free T3 is the weakest of the three tests. It falls in any illness and plays no role in diagnosing an underactive thyroid.
• No trial shows that screening adults without symptoms improves health. A US task force found the evidence insufficient.

References

1. Garber JR, et al. Thyroid. 2012;22:1200-1235 (ATA/AACE guideline on hypothyroidism in adults).
2. Ross DS, et al. Thyroid. 2016;26:1343-1421 (ATA guideline on hyperthyroidism).
3. Bekkering GE, et al. BMJ. 2019;365:l2006 (guideline on treating subclinical hypothyroidism).
4. Feller M, et al. JAMA. 2018;320:1349-1359.
5. LeFevre ML, et al. Ann Intern Med. 2015;162:641-650 (USPSTF statement on thyroid screening).