TSH: what it means.
High TSH suggests an underactive thyroid; low suggests overactive. Conversion needs selenium + iodine.
Written to our editorial standards · reviewed against published research· Updated 2 Jun 2026
Typical reference bands (mIU/L)
Lowunder 0.4 mIU/L
Optimal1-2.5 mIU/L
Highover 4.5 mIU/L
Ranges vary by laboratory, age, and sex, your lab's own reference range always takes precedence.
Common causes
- An underactive thyroid or Hashimoto's raises TSH
- An iodine or selenium shortfall
- An overactive thyroid or over-replacement lowers TSH
What it can feel like
- High TSH: fatigue, weight gain, cold intolerance, dry skin, low mood
- Low TSH: palpitations, anxiety, heat intolerance, weight loss
Who should test
Worth checking with unexplained fatigue, weight change, or cold intolerance. An abnormal TSH should be confirmed and managed with a clinician, usually with free T4 and antibodies.
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Common questions
What does a low TSH mean?
Common drivers include an underactive thyroid or Hashimoto's raises TSH; an iodine or selenium shortfall; an overactive thyroid or over-replacement lowers TSH. A result outside the optimal range is best read in context: discuss it with your clinician, who can weigh the full picture and your lab's own reference range.
How is TSH best supported?
Through diet, lifestyle, and addressing the underlying cause with your clinician. Upload your labs to suppdoc for a tailored read.
Educational use only, not medical advice or diagnosis. Always interpret lab results with a qualified clinician.