TSH explained: why it goes up when the thyroid slows down

TSH is the most commonly ordered thyroid test, and it confuses a lot of people. A high TSH means an underactive thyroid. A low TSH means an overactive one. It feels backwards until you know what TSH actually is.

TSH is a messenger, not a thyroid hormone

TSH stands for thyroid stimulating hormone. It is made by the pituitary gland at the base of the brain, not by the thyroid itself. Its job is to tell the thyroid, a butterfly-shaped gland in the front of the neck, how hard to work.

The thyroid responds by making two hormones, T4 and T3. These control how fast your body runs: your energy, heart rate, body temperature, weight, bowel habit and much more.

The feedback loop

The pituitary constantly checks how much thyroid hormone is in the blood and adjusts TSH to match, much like a thermostat controls a geyser.

• If thyroid hormone levels drop, the pituitary releases more TSH to push the thyroid harder. TSH goes up.
• If thyroid hormone levels rise too high, the pituitary eases off. TSH goes down.
• When everything is balanced, TSH sits in the middle of the range.

So a raised TSH is the pituitary shouting at a thyroid that is not keeping up. A suppressed TSH is the pituitary going quiet because there is already too much thyroid hormone around.

Why TSH is such a sensitive test

The pituitary reacts strongly to small changes. A slight fall in thyroid hormone, too small to move free T4 outside its range, can produce a clear rise in TSH. That makes TSH the best single screening test for thyroid problems, and it often picks up a thyroid that is starting to struggle before symptoms appear.

The range LifePrint uses

TSH is measured in mIU/L. LifePrint uses the range printed by your lab, because it is specific to the assay that produced your result. When no range is printed, LifePrint uses the American Thyroid Association reference interval of 0.4 to 4.0 mIU/L.

Two zones just outside this range are reported as a grey zone rather than simply high or low: a TSH between 4 and 10 mIU/L, and a TSH between 0.1 and 0.4 mIU/L. These are the subclinical zones, where the thyroid may be starting to under or over perform while free T4 is still normal. A TSH of 10 or above, or below 0.1, is reported as clearly abnormal.

Things that shift TSH

• Time of day. TSH is highest overnight and in the early morning, and lower in the afternoon.
• Recovery from illness. TSH can rise for a while as the body recovers from a serious illness or operation.
• Pregnancy. TSH usually falls in early pregnancy, and pregnancy uses its own ranges. Tell your doctor if you are pregnant or planning a pregnancy.
• Age. TSH tends to drift up with age, and a mildly raised level is common and often harmless in older adults.
• Medicines, including steroids, amiodarone, lithium and thyroid medication itself.
• Biotin. High-dose biotin can distort many thyroid tests. Pause it for 48 hours before your draw, as covered in our guide to preparing for a blood test.

When TSH is not enough

TSH relies on the pituitary working normally. In rare cases, the problem is in the pituitary itself, and TSH can look normal or low even though the thyroid is underactive. TSH also cannot tell you why the thyroid is struggling.

This is why the LifePrint Thyroid panel adds free T4 and free T3, the hormones the thyroid actually makes, and TPO antibodies, the most common cause of an underactive thyroid. Together they show where a problem sits and what might be behind it. The Essentials and Energy and Nutrients panels include TSH alone as a screening test.

What happens next

Your LifePrint report explains your TSH in the context of your other results and flags how urgently to follow up. A result in the grey zone rarely needs urgent action and usually calls for a repeat test. A clearly abnormal result, or any abnormal result with symptoms, should be discussed with a registered healthcare practitioner, who can confirm it and advise on next steps.