TSH stands for thyroid-stimulating hormone. It is the most common first test of how your thyroid is working, and the way it moves can seem back-to-front. Here is how to read it.
TSH is made by the pituitary gland in your brain, and its job is to tell your thyroid how much thyroid hormone to produce. Measuring TSH is a common way to check whether your thyroid is keeping up. The relationship can feel counter-intuitive, so it is worth reading carefully.
When the thyroid is underactive and not making enough hormone, the pituitary produces more TSH to push it, so a high TSH often points toward an underactive thyroid. When the thyroid is overactive, the pituitary makes less TSH, so a low TSH often points toward an overactive thyroid. Exact reference ranges vary by lab, and results are usually confirmed with additional thyroid tests.
An abnormal TSH is a signal, not a final answer. Your clinician typically follows it with tests such as free T4, and considers your symptoms and history, before reaching any conclusion. Some medicines and conditions can also affect TSH.
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Decode my labsContent last reviewed 9 July 2026. Reference ranges vary by laboratory; always read your results against the range printed on your own report.
This page explains a common lab marker in general terms using public information from the U.S. National Library of Medicine and other NIH sources. It is educational and is not a diagnosis or medical advice. Reference ranges differ between laboratories and the meaning of a result depends on your age, sex, medications, and overall health. Always discuss your own results with the clinician who ordered them.