Lab marker · Low

Low Thyroid Stimulating Hormone (TSH): what it means

A Thyroid Stimulating Hormone (TSH) result below the normal range is considered low. Here is what a reduced Thyroid Stimulating Hormone (TSH) can point to, and when it is worth discussing with a doctor.

Reference range: 0.4–4.0 mIU/L (most labs); pregnancy and treatment targets differ

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What a low Thyroid Stimulating Hormone (TSH) 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).

Conditions this can relate to

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Mediora.AI is informational - interpretation, not a diagnosis. A single out-of-range value is common and often benign; always discuss decisions with your doctor.