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Biomarker library / Thyroid

TSH (thyroid-stimulating hormone) (TSH)

Also called thyrotropin, thyroid stimulating hormone

TSH is the pituitary's signal to the thyroid. When thyroid hormone runs low, the pituitary raises TSH to push the gland harder; when thyroid hormone runs high, TSH falls. Because the pituitary responds to small shifts, TSH moves earlier and further than the thyroid hormones themselves, which is why it is the first-line thyroid test.

Written by the VitalDecades editorial team Reviewed by the editorial review process Updated

01  /  Reference ranges

What counts as normal

Scale in mIU/L. Cut points from the sources listed below.
Population Adults
Category Reference interval (if your lab prints none)
Range 0.45 to 4.12 mIU/L NHANES III-derived fallback; the guideline says to prefer the laboratory's own interval and notes the 4.12 upper limit is not universally accepted.
Source Clinical practice guidelines for hypothyroidism in adults (ATA/AACE 2012)

Effective

Population Adults
Category Below reference: possible overactive thyroid
Range under 0.45 mIU/L Usually means excess thyroid hormone; needs free T4/T3 and clinical review.
Source Clinical practice guidelines for hypothyroidism in adults (ATA/AACE 2012)

Effective

Population Adults, free T4 normal
Category Subclinical zone: individualized decision
Range above 4.12 and 10 or lower mIU/L Treatment considered with symptoms, positive TPO antibodies, or cardiovascular disease and its risk factors.
Source Clinical practice guidelines for hypothyroidism in adults (ATA/AACE 2012)

Effective

Population Adults
Category Treatment generally considered
Range 10 or higher mIU/L The guideline links TSH above 10 to increased risk of heart failure and cardiovascular mortality.
Source Clinical practice guidelines for hypothyroidism in adults (ATA/AACE 2012)

Effective

Normal vs optimal

Your lab's printed interval comes first. When a third-generation assay's limits are not available, the guideline says to use 0.45 to 4.12 mIU/L, a range derived from NHANES III; it also notes that the 4.12 upper limit has not been universally accepted, and TSH drifts upward with age. There is no published "optimal" TSH inside the reference range, so we do not print one.

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02  /  Why it matters

What the number tells you

A high TSH most often means an underactive thyroid (hypothyroidism); a low TSH usually means an overactive one (hyperthyroidism). The ATA/AACE guideline distinguishes a subclinical zone, where TSH is above the reference range but free T4 is still normal, from overt disease. Above 10 mIU/L the guideline notes increased risk of heart failure and cardiovascular mortality and says treatment should be considered; between the upper reference limit and 10, treatment is individualized around symptoms, TPO antibodies and cardiovascular risk.

03  /  In practice

Testing and what moves it

When it is tested

  • When symptoms suggest an under- or overactive thyroid (fatigue, weight change, cold or heat intolerance, palpitations, hair or skin changes).
  • To titrate levothyroxine: the guideline re-checks TSH about 4 to 8 weeks after a dose change.
  • TSH alone can mislead in pituitary disease and in the first trimester of pregnancy; free T4 and context matter.

What raises it

  • An underactive thyroid, most commonly autoimmune (Hashimoto) thyroiditis.
  • Recovery from illness, some medications (lithium, amiodarone), thyroid surgery or radioiodine.

What lowers it

  • An overactive thyroid (Graves disease, toxic nodules, thyroiditis in its early phase).
  • Too high a levothyroxine dose; biotin supplements can distort the assay itself.

04  /  Related

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Frequently asked questions

What is a normal TSH level?

Use your lab report's printed interval first. When one is not available, the ATA/AACE guideline's fallback is 0.45 to 4.12 mIU/L, derived from NHANES III. TSH also rises modestly with age, so a value slightly above range means something different at 75 than at 40.

My TSH is 5.5 but I feel fine. Do I need medication?

Not necessarily. With a normal free T4 that is the subclinical zone, and the guideline individualizes treatment below 10 mIU/L around symptoms, TPO antibodies and cardiovascular risk. It is a conversation with your clinician, usually starting with a repeat test.

What TSH level needs treatment?

Above 10 mIU/L the ATA/AACE guideline says patients should be considered for levothyroxine, citing increased risk of heart failure and cardiovascular mortality. Below 10, treatment is individualized.