TSH

Thyroid-stimulating hormone (TSH) is the pituitary signal that drives thyroid hormone production and the first-line screen for thyroid dysfunction. Because of the inverse logarithmic relationship between TSH and free T4, even small thyroid hormone changes produce large TSH shifts, which makes it sensitive but sometimes oversensitive. A reading just outside the reference range without symptoms usually warrants a repeat draw rather than immediate treatment.

Optimal range

Range varies by individual.

Test frequency

Annually as routine screening; every six to eight weeks during dose titration of thyroid medication or pursuit of an unexplained value.

When to measure

Measure as part of every annual physical from age 30 onward; earlier if there is family history of thyroid disease or autoimmune conditions. Re-measure six to eight weeks after starting or changing any thyroid medication, since that is the time required for a new steady state. Always re-measure any abnormal value before acting on it, single-draw variability is substantial, and TSH responds transiently to acute stress, illness, and time of day.

How to measure

Standard serum immunoassay; nearly every lab in the world offers it. Cost is $15–$50 retail; included in most basic annual physical panels. No fasting required. Morning draws are conventionally preferred but evidence for this is weak, consistency of timing across measurements matters more than the absolute time of day. Pair with free T4 (always), free T3 (if symptomatic), and TPO/TG antibodies (if elevated) for a complete picture.

Why this biomarker matters

Conventional laboratory reference ranges (typically 0.45–4.5 mIU/L) are wide and population-derived. Many thyroid-experienced clinicians target a narrower optimal window of 0.5–2.5 mIU/L for energy, metabolic rate, exercise tolerance, and cognitive function, particularly in younger adults. Subclinical hypothyroidism, elevated TSH with normal free T4, sustained across multiple measurements is associated in observational data with worse lipid profiles, slower metabolic rate, and modestly higher long-term cardiovascular risk. Both directions matter. Subclinical hyperthyroidism (suppressed TSH with normal free T4) associates with increased risk of atrial fibrillation and reduced bone density over decades, particularly in postmenopausal women. The clinical value of TSH is not the single number but the trend: TSH that is steadily climbing into the upper reference range, even without crossing the cutoff, warrants attention to iodine, selenium, iron status and to the possibility of early autoimmune thyroiditis (Hashimoto's),best confirmed with TPO and thyroglobulin antibodies.

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