TSHThyroid-stimulating hormone
The brain's signal that controls the thyroid: the first-line thyroid check.
Thyroid stimulating hormone is released by the pituitary gland to tell the thyroid how much thyroid hormone to make. It is the first test used to assess thyroid function, and the relationship it captures is inverse: when thyroid hormone is low, TSH rises to push harder; when thyroid hormone is high, TSH falls.
That inversion trips people up regularly. A high TSH suggests an underactive thyroid, not an overactive one, because it is a measure of the demand signal rather than the output.
It is used first because it is exquisitely sensitive. The pituitary responds to small changes in thyroid hormone, so TSH moves before free T4 does, which is why a normal TSH usually makes significant thyroid dysfunction unlikely in someone without symptoms.
It varies more than a single result suggests. TSH has a daily rhythm, higher overnight and in the early morning, and it shifts with illness, pregnancy, biotin supplements interfering with the assay, and several medications. Diagnosis is therefore made on repeated tests, usually with free T4 alongside.
Subclinical hypothyroidism, a mildly raised TSH with normal thyroid hormone, is common, often transient, and one of the more contested areas in endocrinology, with genuine disagreement about when treatment helps.
Thyroid symptoms overlap heavily with the rest of life: fatigue, weight change, low mood, feeling cold, dry skin. That overlap is why the test is ordered often and why a normal result in someone tired is a useful piece of information rather than a dead end.
In practice, take an abnormal TSH to a GP for proper interpretation rather than to a supplement labelled thyroid support, and mention any biotin you take before the blood is drawn.
Where your own result would sit
Most people reading this page are holding a blood-test report with this marker on it, and the question they arrived with is whether their number is normal. A reference interval says where most results from a healthy population fall; a guideline threshold is where a clinical body draws a line for a decision. Neither is a target, and a result on the wrong side of one is not a diagnosis: the same number can be unremarkable in one person and worth looking into in another, which is why the clinician who ordered the test reads it against your history rather than against a list.
TSH has no published interval on this page yet. We hold one, and the dashboard draws it beside a reader’s own result, but it has not been read back against the laboratory’s own page since it was collected. A figure printed beside somebody’s blood result has to have been seen at its source, not remembered, so this one waits.
On your own blood test?
TSH is one of the markers FitTools can read from an uploaded blood-test report. You confirm every value before anything is saved, and your results are explained together in one place.