TSH
Thyroid stimulating hormone
TSH is the pituitary's instruction to the thyroid. It moves in the opposite direction to thyroid output, which is why a high TSH suggests an underactive thyroid.
Reference range
mIU/L- Women
- 0.5–4.2 mIU/L
- Men
- 0.5–4.2 mIU/L
Reference ranges differ between laboratories and assays. Always read your result against the range printed on your own report.
What TSH measures
TSH is not made by the thyroid. It is made by the pituitary gland in the brain, and its job is to tell the thyroid how hard to work. The pituitary constantly samples how much thyroid hormone is circulating and adjusts its instruction accordingly.
That feedback loop is why TSH reads backwards to intuition. If the thyroid is underperforming, the pituitary shouts louder and TSH goes up. If the thyroid is overproducing, the pituitary goes quiet and TSH falls. A high TSH therefore points to an underactive thyroid, not an overactive one.
Because the loop amplifies small changes, TSH shifts before free T4 does. This sensitivity makes it the standard first-line test — but it also means a single borderline value says less than people assume.
Why it gets tested
To investigate fatigue, unexplained weight change, cold or heat intolerance, hair thinning, irregular periods or low mood. It is also the routine monitoring test for anyone on thyroid hormone replacement.
Commonly associated with low values
- An overactive thyroid, with Graves' disease the most common cause
- Thyroid hormone replacement dosed slightly too high
- Early pregnancy, where hCG mildly stimulates the thyroid
Commonly associated with high values
- An underactive thyroid, most often from Hashimoto's thyroiditis
- Thyroid hormone replacement dosed slightly too low
- Recovery from a serious illness, temporarily
- Iodine deficiency, in regions where it occurs
What can shift the result
- TSH follows a daily rhythm and peaks overnight — morning samples read higher than afternoon ones
- Biotin supplements can distort thyroid assays substantially and should be paused before testing
- Acute illness suppresses TSH and normalises again afterwards
Questions
Why does a high TSH mean an underactive thyroid?
Because TSH is the signal to the thyroid, not the output of it. When the thyroid produces too little hormone, the pituitary compensates by sending a stronger signal — so TSH rises. It is the same logic as pressing harder on an accelerator that is not responding.
Does the time of day matter for a TSH test?
It can. TSH peaks in the early hours and drops through the day, and the difference between an early morning and a mid-afternoon sample is enough to move a borderline result across the reference limit. For monitoring over time, testing at a consistent hour makes the values easier to compare.
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This page is general information about a laboratory value. It is not a diagnosis and does not replace medical advice. Discuss your own results with a doctor, who can read them in the context of your history and symptoms.