What is TSH?
TSH is the hormone your pituitary uses to steer your thyroid. It works as a thermostat: when your thyroid hormone drops, the pituitary turns TSH up to push the thyroid harder; when there is enough, TSH falls. So TSH moves in the opposite direction to your thyroid's output — a high TSH points to an underactive thyroid, a low one to an overactive thyroid. Because small changes produce large swings, TSH is the most sensitive first-line screen. When it is abnormal, free T4 is added to clarify what is happening.
Why is TSH relevant?
Your thyroid sets the pace of your whole metabolism. An under- or over-active gland is common, often goes unnoticed, and is very treatable once found. Symptoms like fatigue, weight changes, feeling cold, low mood, hair thinning, or palpitations fit exactly the picture a single TSH can start to explain. The direction tells the story: a high TSH fits an underactive thyroid, a low one an overactive thyroid. A mildly raised TSH often normalises on its own and is usually watched rather than treated straight away. A markedly abnormal value is more clearly significant and should be reviewed with a clinician alongside free T4.
TSH high or low — what it means
A single TSH is a snapshot. It is highest in the morning and falls through the day, so timing matters when comparing results. Biotin supplements (common in hair and nail products) can distort the reading — pause them a few days before testing. A borderline or abnormal value is best repeated a few weeks later rather than acted on from one reading. A high TSH usually points to an underactive thyroid, often from the autoimmune condition Hashimoto's. A low TSH usually points to an overactive thyroid. TSH is not something you adjust with lifestyle the way you might lower LDL or CRP — treatment is medical. Any persistently abnormal value should be worked up with a doctor.
TSH reference ranges
Cut-offs vary between laboratories and methods and shift with age and pregnancy. Always read your value against the range printed on your own result. TSH is also reported as mU/L or µIU/mL (the same unit).
Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.
Read about our scientific approach →Read the guide: Hormones →