Low tsh

Written to our editorial standards · reviewed against published research· Updated 2 Jun 2026
A medical finding, review with a doctor

A low TSH usually means the thyroid is overactive (hyperthyroidism), which causes palpitations, weight loss, anxiety, and heat intolerance, and needs prompt medical assessment.

When the thyroid produces too much hormone, the brain stops signalling and TSH drops. This speeds the body up, causing a fast or irregular heartbeat, unintended weight loss, tremor, and anxiety. It is a medical condition that needs diagnosis and treatment, not supplements.

Common causes

  • Graves' disease (autoimmune)
  • Thyroid nodules
  • Too much thyroid medication
  • Excess iodine
  • Thyroiditis

Associated symptoms

  • Palpitations or a fast heartbeat
  • Unintended weight loss
  • Anxiety and tremor
  • Feeling hot, sweating
  • Trouble sleeping
What to do

Do not take iodine or thyroid-stimulating supplements. The next step is medical: a doctor will confirm with free T4 and antibodies and arrange treatment.

When to see a doctor

See a doctor promptly for a low TSH, especially with palpitations or weight loss, as an overactive thyroid affects the heart and needs treatment.

Full marker
TSH: ranges, what it measures & both directions
Read the TSH guide →

Common questions

What does low tsh (overactive thyroid) mean?
A low TSH usually means the thyroid is overactive (hyperthyroidism), which causes palpitations, weight loss, anxiety, and heat intolerance, and needs prompt medical assessment. When the thyroid produces too much hormone, the brain stops signalling and TSH drops. This speeds the body up, causing a fast or irregular heartbeat, unintended weight loss, tremor, and anxiety. It is a medical condition that needs diagnosis and treatment, not supplements.
What causes low tsh?
Graves' disease (autoimmune); Thyroid nodules; Too much thyroid medication; Excess iodine; Thyroiditis.
When should I see a doctor?
See a doctor promptly for a low TSH, especially with palpitations or weight loss, as an overactive thyroid affects the heart and needs treatment.

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Educational use only, not medical advice or diagnosis. Reference ranges vary by lab, age, sex, and medication, and a single result is read in context. Always review your results with a qualified clinician.