TSH
The pituitary's signal telling your thyroid to work harder — so high TSH usually means an underactive thyroid.
What it means
TSH is not a thyroid hormone. It is the message your pituitary gland sends telling the thyroid to produce more.
That is why the result reads backwards from what people expect:
- High TSH — the pituitary is shouting, so the thyroid is underactive
- Low TSH — no shouting needed, so the thyroid is overactive
Think of it as a thermostat. A thermostat working hard means the room is cold.
In practice
On any thyroid function test, and on most general health panels.
Usually reported alongside free T4, and sometimes free T3 and thyroid antibodies.
Why it matters
A single mildly abnormal TSH usually needs repeating in a few weeks before anything is started, because it moves with illness, stress and time of day.
If you already take levothyroxine, a suppressed TSH may mean your dose is too high — which matters for bone and heart health, and is worth raising rather than leaving.
Ask about antibodies if the result is borderline; they help predict whether things will progress.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026