Thyroid Stimulating Hormone (TSH)
TSH is the pituitary's signal to the thyroid gland. It is the single most informative thyroid screening test — high TSH suggests an under-active thyroid, low TSH an over-active one.
What it measures
Your pituitary gland releases TSH to tell the thyroid how much hormone to produce. If thyroid output (free T4) is low, TSH rises to push the gland harder; if output is high, TSH drops to ease off. TSH is exquisitely sensitive — small changes in thyroid function move TSH long before T4 falls outside its reference range. That's why nearly every screening lab starts with TSH alone.
What a high value can mean
- Primary hypothyroidism — under-active thyroid (Hashimoto's autoimmune thyroiditis is the most common cause in iodine-replete populations).
- Sub-clinical hypothyroidism — TSH 4–10 with normal free T4, often early Hashimoto's.
- Recovery from non-thyroidal illness — transient elevation as the body re-balances.
- Some medications — amiodarone, lithium.
What a low value can mean
- Hyperthyroidism — Graves' disease, toxic nodular goitre, thyroiditis.
- Over-replacement — too high a levothyroxine dose.
- Central hypothyroidism — pituitary or hypothalamic failure (rare); both TSH and T4 are low.
- Acute illness or starvation — transient suppression.
When to discuss with a doctor
TSH outside the 0.4–4.0 mIU/L band — especially if paired with fatigue, weight change, palpitations or mood changes — warrants follow-up labs (free T4, sometimes free T3 and thyroid antibodies) and a clinician visit. In pregnancy, target ranges are tighter (typically <2.5 mIU/L in the first trimester).