Normal TSH is generally 0.4–4.5 mIU/L, though ranges vary slightly by laboratory. Some experts advocate a narrower range of 0.5–2.5 mIU/L. TSH is inversely related to thyroid hormone levels — high TSH means low thyroid function and vice versa.

TSH is the most sensitive indicator of thyroid status because small changes in thyroid hormone levels cause amplified changes in TSH. TSH abnormalities often precede free T4 changes, making it better for detecting early (subclinical) thyroid disease.

Subclinical hypothyroidism: elevated TSH with normal free T4 (affects 4–10% of adults). Subclinical hyperthyroidism: suppressed TSH with normal free T4 (affects 1–2%). Both may progress to overt disease and should be monitored. Treatment depends on TSH level and symptoms.

Medications (levothyroxine, biotin supplements, steroids, lithium), pregnancy (lower TSH in first trimester), time of day (TSH peaks at night), illness (non-thyroidal illness syndrome), and pituitary disorders can all affect TSH. Biotin supplements can falsely alter TSH assays — stop 2 days before testing.