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TSH

The hormone the pituitary gland releases to tell the thyroid to work harder. Because the pituitary reacts to thyroid hormone levels, TSH moves in the opposite direction to thyroid activity — it goes up when the thyroid is underactive and down when it is overactive. That inverse relationship is the single most confusing thing about this test.

General information only — not a diagnosis, not treatment and not a substitute for your doctor. Reference ranges differ between laboratories, so the range printed on your own report is the one that counts, and only a clinician who knows your history can say what a result means for you. Never start, stop or change a medicine or supplement based on this page.

Usual adult reference range

The ranges below are typical adult reference intervals. Your own laboratory sets its own, printed next to your result, and that is the one to compare against.

Source: Typical adult reference interval published by clinical laboratories; assay-specific and pregnancy-specific intervals differ. · checked 2026-09-02

What a high result generally indicates

A raised TSH suggests the pituitary is pushing an underactive thyroid. Whether that matters depends on how high it is and on the free T4 level measured alongside it; a mildly raised TSH with normal T4 is common, often transient, and usually repeated before anything is concluded.

What a low result generally indicates

A suppressed TSH suggests the thyroid is producing more hormone than the body needs, or that thyroid hormone is being taken in a dose above requirement. It is also seen in the first trimester of pregnancy and in serious non-thyroid illness.

These are general patterns, not a diagnosis. Many things cause a high or low result, and the same figure means different things in different people. What yours means is a question for the doctor who ordered the test.

What affects this result

• Time of day — TSH is higher at night and in the early morning.
• Any serious illness distorts thyroid tests; testing is usually deferred.
• Biotin supplements interfere with many thyroid assays, in either direction.
• Pregnancy has its own trimester-specific reference intervals.
• Levothyroxine dose and timing relative to the blood draw.
• Amiodarone, lithium and some other medicines alter thyroid function.

Before the test

No fasting needed. High-dose biotin is usually stopped for a couple of days beforehand on the laboratory's advice.

Any preparation instruction that comes with your test slip overrides this general note — follow the one your clinician or laboratory gave you.

The one thing to take from this page

The range above is a typical interval for orientation. Your own laboratory's range, printed beside your result, is the one that applies to you — and a result is only meaningful alongside your symptoms, your history, your medicines and your other tests. That reading is a clinician's job. This page cannot diagnose anything, cannot tell you to change a medicine or a supplement, and is not a substitute for the person who ordered your test.

Frequently asked questions

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