A TSH test measures thyroid stimulating hormone, a hormone made by the pituitary gland at the base of the brain. The pituitary senses how much thyroid hormone is circulating, then releases TSH to tell the thyroid to work harder or ease off. Because of that feedback loop, TSH reports the pituitary's assessment of your thyroid rather than the amount of thyroid hormone in your blood.

Why TSH moves in the opposite direction to thyroid hormone

TSH runs backwards compared with what most people expect. When the thyroid makes too little hormone, the pituitary pushes harder and TSH goes up. When the thyroid makes too much, the pituitary backs off and TSH goes down. A high TSH usually means an underactive thyroid. A low TSH usually means an overactive one. The relationship is also amplified: a small fall in free T4 produces a proportionally larger rise in TSH, which is why TSH is sensitive enough to be the first test ordered.

What a TSH result cannot tell you

TSH cannot tell you why a thyroid is failing. It does not separate Hashimoto's thyroiditis from thyroid surgery, from amiodarone or lithium, or from iodine excess. TSH is also unreliable when the pituitary itself is damaged, because a damaged pituitary cannot raise TSH even when thyroid hormone is genuinely low. The American Thyroid Association makes this point directly in its patient guidance on hypothyroidism. Acute illness, high-dose biotin supplements and some medications can distort the number as well.

Reading TSH together with free T4

TSH is interpreted alongside free T4, the fraction of thyroid hormone that is unbound and available to tissues. The pair of results is what names the pattern.

Pattern	TSH	Free T4	Usual meaning
Overt hypothyroidism	High	Low	Thyroid is failing
Subclinical hypothyroidism	High	Normal	Mild or early underactivity
Overt hyperthyroidism	Low	High	Thyroid is overactive
Subclinical hyperthyroidism	Low	Normal	Mild overactivity
Central hypothyroidism	Normal or low	Low	Pituitary or hypothalamic problem

If you are having TSH checked for the first time, ask for free T4 on the same draw, and ask whether TPO antibodies are worth adding. Keep your results from the same laboratory where you can, because reference ranges differ between assays and comparing numbers across labs can mislead you into thinking something changed when only the method did. If you take biotin, stop it at least two days before the blood draw.

TSH and free T4 are related log-linearly, so free T4 falling within its own reference interval can still drive TSH several-fold higher. In the NHANES III disease-free reference population of 13,344 people, the geometric mean TSH was 1.40 mIU/L and 11.3 percent were TPO antibody positive.

Third-generation TSH assays have a functional sensitivity near 0.01 mIU/L, which is what allows subclinical hyperthyroidism to be detected. Most current immunoassays use a streptavidin-biotin capture step, so supplemental biotin can falsely lower measured TSH and falsely raise free T4, mimicking Graves' disease. The American Thyroid Association reports that biotin use can result in falsely high levels of T4 and T3 and falsely low levels of TSH, and advises stopping biotin for at least two days before thyroid testing. Non-thyroidal illness suppresses TSH acutely and can produce a rebound rise during recovery, so inpatient or immediately post-illness TSH values should be repeated once the patient is well.

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Why TSH moves in the opposite direction to thyroid hormone

What a TSH result cannot tell you

Reading TSH together with free T4

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