Thyroid Stimulating Hormone (TSH)

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.

• Adults: about 0.4-4.0 mIU/L for most labs.
• Pregnancy uses lower, trimester-specific limits.
• TSH moves opposite to thyroid hormone: a high TSH suggests an underactive thyroid, a low TSH an overactive one.
• It is read together with free T4 (and sometimes free T3).

Your lab's printed range is what counts.

• 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.

• 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.

• High TSH (underactive thyroid): fatigue, cold intolerance, weight gain, constipation, dry skin, low mood and heavy or irregular periods.
• Low TSH (overactive thyroid): weight loss, palpitations, heat intolerance, anxiety, tremor and loose stools.
• Mildly abnormal TSH is often symptom-free and simply repeated with free T4 before any treatment.