TSH is the pituitary gland's steering signal to the thyroid, and it moves in the opposite direction to thyroid hormone levels. Its exquisite sensitivity makes it the single best first test for thyroid function.
Standard range versus optimal range
| Framework | TSH value | Source |
|---|---|---|
| Standard lab reference | about 0.4-4.5 mIU/L | Common laboratory range |
| Optimal for most adults | roughly 0.5-2.5 mIU/L | Thyroid outcome studies |
| Pregnancy, first trimester | below 2.5 mIU/L target | ATA pregnancy guideline |
When it runs high
High TSH means the pituitary is pushing harder because thyroid hormone output is too low. Modest rises with normal free T4 define subclinical hypothyroidism, common with age, while higher values with low free T4 confirm overt disease.
- Hashimoto autoimmune thyroiditis, the leading cause
- Iodine deficiency or excess
- Thyroid surgery or radioactive iodine treatment
- Lithium and amiodarone
- Recovery phase of thyroiditis
- Poor absorption of levothyroxine in treated patients, often from taking it with food or coffee
When it runs low
Low TSH means the pituitary is braking because thyroid hormone is plentiful. Values suppressed below 0.1 mIU/L carry risks of atrial fibrillation and bone loss, especially in older adults, even when free hormones remain technically normal.
- Graves autoimmune hyperthyroidism
- Toxic nodules or multinodular goiter
- Thyroiditis with hormone leak
- Too much thyroid medication
- Central hypothyroidism, rare, where both TSH and free T4 run low
- Pregnancy first trimester, a mild physiological dip
The thermostat of the thyroid axis
The pituitary samples circulating thyroid hormone constantly and adjusts TSH in response, and because the relationship is log-linear, a halving of free T4 can double or triple TSH. That amplification is why TSH detects thyroid problems years before hormone levels leave their reference ranges. It is also why the test fails in rare pituitary disease, where TSH stays normal despite low hormone, a situation that demands free T4 measurement whenever symptoms and TSH disagree.
How much does the upper limit matter
Whether the healthy ceiling is 2.5 or 4.5 mIU/L has been argued for two decades, because subclinical hypothyroidism between those values sometimes progresses and sometimes resolves. Treatment trials found that patients under 65 with TSH persistently above 10 clearly benefit from levothyroxine, while values between 4.5 and 10 call for an individualized decision based on symptoms, antibodies, pregnancy plans and cardiovascular risk. For longevity, values in the low-normal half of the range associate with the most favorable metabolic profile.
Testing it properly
TSH follows a daily rhythm, peaking overnight and dipping in the afternoon, so a borderline result deserves a morning repeat before any conclusion. Biotin supplements in hair and nail doses can falsely lower TSH and raise free T4 on many analyzers, and stopping them two to three days before the draw prevents a classic artifact. Acute illness suppresses TSH transiently, another reason borderline results get repeated in six to twelve weeks rather than treated immediately.
Related calculators
Related biomarkers Free T4 · Free T3 · TPOAb