Quick Guide: Thyroid-stimulating hormone (TSH) Blood Test
What it is: A TSH (thyroid-stimulating hormone) test measures the level of TSH, a pituitary hormone that signals the thyroid gland to produce thyroid hormone, in the blood.
Why it's ordered: Usually the first test ordered to screen for or monitor thyroid conditions like hypothyroidism or hyperthyroidism, based on symptoms such as fatigue, weight changes, or heart rate changes.
What it detects: TSH rises when thyroid hormone (T4) is too low, as the pituitary tries to stimulate more production, and falls when thyroid hormone is too high, making TSH a sensitive first-line marker of thyroid function.
Collection & prep: A standard blood draw from a vein in the arm. No special preparation is typically required.
Results:
High – suggests an underactive thyroid (hypothyroidism)
Low – suggests an overactive thyroid (hyperthyroidism)
Normal – thyroid function appears balanced; if abnormal, follow-up free T4 (and sometimes T3) testing is typically added
Possible complications of thyroid dysfunction: Untreated hypothyroidism or hyperthyroidism can affect heart rate, weight, mood, and energy levels; TSH results are followed up with T4/T3 testing and clinical evaluation before starting treatment.
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