TSHThyroid-stimulating hormone
The brain's signal that controls the thyroid — the first-line thyroid check.
TSH (thyroid-stimulating hormone) is released by the pituitary gland in the brain to tell the thyroid how much thyroid hormone to make. The thyroid, in turn, sets much of the body's metabolic rate.
It matters because TSH is the first-line screen for thyroid problems, and it works by feedback: a high TSH usually means the thyroid is underactive (the brain is shouting for more), while a low TSH suggests an overactive thyroid. Thyroid dysfunction is a common, treatable cause of fatigue, weight change and low mood.
In practice, an abnormal TSH is followed up with thyroid hormone levels (such as free T4) to confirm the picture, since TSH alone does not tell the whole story.
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Written and reviewed by Mathew Beale, MSc Biotechnology, University of Reading.
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