thyroid-stimulating hormone · thyrotropin
TSH: what your thyroid result means
TSH is the pituitary's instruction to the thyroid, which is why it moves in the opposite direction to the problem. A high TSH means an underactive thyroid, not an overactive one.
What it measures
TSH is made by the pituitary gland and tells the thyroid how much hormone to produce. When thyroid hormone runs low the pituitary shouts louder and TSH rises; when it runs high, TSH is suppressed. That feedback is very sensitive, so TSH usually moves before free T4 does.
Why it gets tested
To investigate tiredness, weight change, cold or heat intolerance, hair and skin changes, palpitations, low mood, constipation and irregular periods — and to monitor anyone on levothyroxine or being treated for an overactive thyroid. It is also checked in pregnancy, in infertility, and where thyroid or autoimmune disease runs in the family.
Typical ranges
Most laboratories use roughly 0.4–4.0 mIU/L, though the upper limit varies and is lower in the first trimester of pregnancy. TSH is not steady through the day — it peaks overnight and dips in the afternoon — so two borderline results taken at different times are not really comparable.
Compare against the range printed on your own report, not this one. Laboratories differ, and yours is the one your doctor is reading.
What a high result can mean
An underactive thyroid, most commonly autoimmune. A raised TSH with a normal free T4 is subclinical hypothyroidism: common, often mild, and not always needing treatment. TSH also rises while recovering from serious illness, on lithium or amiodarone, and when a levothyroxine dose is too small — or is being taken with food, calcium, iron or a stomach-acid tablet, all of which block its absorption.
What a low result can mean
An overactive thyroid, or replacement set too high. It also falls normally in the first trimester of pregnancy, and during serious non-thyroidal illness. A suppressed TSH with normal hormones is subclinical hyperthyroidism, which matters mainly for bone density and heart rhythm over time.
What moves it
Time of day, pregnancy, acute illness, and several drugs. Biotin — a common hair-and-nail supplement — distorts many thyroid assays, and stopping it for a couple of days before the test avoids a great deal of confusion. How and when you take levothyroxine changes the result without anything having changed in the gland.
What usually happens next
An abnormal TSH is usually repeated with free T4 — and free T3 where an overactive thyroid is suspected — after six to eight weeks rather than immediately, because a single reading catches too many passing changes. Thyroid antibodies are added when autoimmune disease is likely, and an ultrasound when the gland feels enlarged or lumpy.
Common questions
Why does a high TSH mean an underactive thyroid?
Because TSH is the signal, not the hormone. When the thyroid under-produces, the pituitary raises TSH to push it harder — so the number goes up while the gland does less. It is the reverse of what most people expect.
Should I take my levothyroxine before the blood test?
Most guidance is to have the blood taken before that day's dose, since taking it just beforehand raises the measured hormone. What matters most is consistency — the same routine each time, so one result can be compared with the last.