What does a high TSH mean?

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TSH - thyroid-stimulating hormone - is the pituitary's instruction to the thyroid: make more hormone. When the thyroid underperforms, the pituitary shouts louder, so a high TSH signals an underactive thyroid - and counterintuitively, high TSH means low thyroid function. NIDDK's hypothyroidism guidance uses TSH as the front-line test. The reading depends on the companion test: high TSH with low free T4 is overt hypothyroidism, usually treated with levothyroxine; high TSH with normal T4 is subclinical hypothyroidism - mild, common, and managed by watching or treating depending on how high the TSH runs, symptoms, pregnancy plans, and antibody status.

Symptoms when it matters: fatigue, weight gain, feeling cold, constipation, dry skin, hair thinning, low mood, and slow heart rate - the whole system running at low revs. Symptoms plus a rising TSH strengthens the case for treatment.

Before any label: repeat the test. TSH wobbles with illness, time of day, biotin supplements - stop those two days before testing - and some medications, and guidelines call for confirming an abnormal result. Untreated hypothyroidism matters most in pregnancy, where thyroid hormone is critical early - high TSH with a positive pregnancy test is a prompt call. And TSH creeping up over years deserves tracking even when symptoms are absent.

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