Thyroid-Stimulating Hormone

TSH is made by the pituitary gland, not the thyroid. It instructs the thyroid to produce hormone, and it is switched off by the hormone it produces — a feedback loop. Counter-intuitively, this makes a pituitary hormone the single best first-line test of thyroid function.

The relationship between TSH and thyroid hormone is log-linear: a small fall in thyroid hormone produces a large rise in TSH. That amplification makes TSH exquisitely sensitive to early thyroid dysfunction — it moves before the thyroid hormones themselves leave the normal range. A high TSH means the pituitary is shouting at an underperforming thyroid; a low TSH usually means there is too much thyroid hormone around.

1. 1

   FoundationHormonal HealthHow to read hormones properly — and why both over-treatment and under-treatment cause harm.
2. 2

   ConditionHypothyroidismAn underactive thyroid
3. 3

   Signal RecordLevothyroxineSynthetic T4 replacement for hypothyroidism
4. 4

   Body SystemHormonesEndocrine signaling that regulates the whole body.
5. 5

   BiomarkerFree T4The unbound thyroid hormone — read alongside TSH

1. Measure it
2. Understand your result
3. See what raises and lowers it
4. Apply evidence-based interventions
5. Recheck on your review cadence

An orientation to how this topic is typically approached — not medical advice.

TSH is read together with free T4, never alone. High TSH with low free T4 is overt hypothyroidism and warrants treatment. High TSH with normal free T4 is subclinical — and in older adults, randomised evidence found no symptom benefit from treating it. Low TSH suggests thyroid hormone excess, which may be from disease or from too much levothyroxine. If the pituitary itself is diseased, TSH becomes unreliable and the entire interpretive logic reverses.

• Serum TSH (first-line)
• Free T4 (always interpreted alongside TSH)
• TPO antibodies (to identify autoimmune cause)

TSH is the validated, guideline-endorsed first-line test of thyroid function, with the log-linear TSH/T4 relationship making it highly sensitive to early dysfunction. Its main limitations are well characterised: unreliability in acute illness and pituitary disease, biotin assay interference, and the frequent over-treatment of a raised TSH with a normal free T4.

• ATA Guidelines for the Treatment of Hypothyroidism
• TRUST trial (levothyroxine in subclinical hypothyroidism)

Sources are named authoritative guidelines and scientific bodies. Biomarker pages present established clinical context; calibrated evidence verdicts on specific interventions live in the linked Signal Records.