TSH is produced by the pituitary gland and acts as the body’s primary thyroid regulator. When thyroid hormone levels fall, the pituitary produces more TSH to stimulate the thyroid; when levels are high, TSH falls. This inverse relationship makes TSH uniquely sensitive for detecting thyroid dysfunction.
A raised TSH indicates the pituitary is working hard to compensate for an underactive thyroid — pointing to hypothyroidism or early Hashimoto’s thyroiditis. A suppressed TSH suggests the thyroid is overactive and the pituitary has backed off — consistent with hyperthyroidism.
Standard reference ranges are approximately 0.4–4.0 mIU/L in most laboratories, though labs vary. The interpretation is more nuanced than a binary normal/abnormal: a TSH of 3.8 mIU/L may be clinically relevant in a woman with persistent fatigue and elevated TPO antibodies, while a TSH of 4.2 mIU/L may be appropriate in an elderly person without symptoms.
Pregnancy significantly alters TSH targets: trimester-specific ranges are lower (first trimester upper limit often 2.5 mIU/L in guidelines), because adequate thyroid hormone is critical for foetal brain development during the first 12 weeks. All women with known thyroid disease — or Hashimoto’s thyroiditis — should have TSH checked early in pregnancy and monitored throughout.
TSH is a lab value; treatment decisions are made on the full clinical picture with your clinician.