Biomarker guide
TSH (µIU/mL)
Thyroid-stimulating hormone (TSH) measures the pituitary signal that tells the thyroid how much hormone to make, making it the standard first-line thyroid screening test. People test it to check whether the thyroid is keeping pace with the body’s needs, since both fatigue-like and overactive symptoms can trace back to thyroid function.
Reference ranges
Commonly used adult reference ranges. Labs vary in methodology and cutoffs, interpret your results against your lab report's stated ranges and with your doctor.
Commonly used reference ranges run roughly 0.4–4.0 µIU/mL; TSH moves inversely to thyroid activity, and time of day, illness, and biotin can skew results.
| Zone | Range |
|---|---|
| Low | 0–0.4 µIU/mL |
| Normal range | 0.4–4 µIU/mL |
| Mildly elevated | 4–6 µIU/mL |
| High | 6–10 µIU/mL |
The importance of TSH
TSH works like a thermostat reading: when thyroid hormone runs low, the pituitary raises TSH to demand more; when hormone is plentiful, TSH falls. This inverse logic trips people up, a high TSH suggests an underactive thyroid, and a low TSH suggests an overactive one.
Because the pituitary is exquisitely sensitive, TSH often shifts before thyroid hormone levels themselves leave their ranges, which is why it is the usual screening entry point. Mildly elevated TSH with normal Free T4, sometimes called subclinical hypothyroidism, is common, particularly in women and with age, and is often simply monitored rather than treated.
TSH also has real day-to-day variability: it runs higher in the early morning, rises modestly in winter, and can be skewed by acute illness, biotin supplements, and certain medications. An out-of-range result is usually repeated, often with Free T4 and Free T3 added, before any decisions are made, a sequence worth walking through with your doctor.
Frequently asked questions
What is a normal TSH level?
Commonly used reference ranges for TSH run from about 0.4 to 4.0 µIU/mL, though the exact bounds vary by lab and some clinicians use tighter targets in specific situations such as pregnancy. A result outside the range is usually repeated and paired with Free T4 before conclusions are drawn.
Why is high TSH associated with an underactive thyroid?
Because TSH is the demand signal, not the supply. When the thyroid produces too little hormone, the pituitary compensates by raising TSH to push for more, so an elevated TSH usually means the thyroid is underdelivering, while a suppressed TSH suggests hormone excess.
Can TSH fluctuate without a thyroid problem?
Yes. TSH runs higher in the early morning and in colder months, rises transiently during recovery from illness, and can be distorted by biotin supplements and medications such as steroids. This variability is why a single mildly abnormal result is typically rechecked in 6–12 weeks.
Should I test more than TSH?
TSH alone is a reasonable screen, but adding Free T4, and sometimes Free T3 or thyroid antibodies, shows whether the thyroid is actually delivering hormone and whether an autoimmune process is present. Which combination makes sense for your situation is worth discussing with your doctor.
Panels that measure TSH
Thyroid Function Blood Test Panels: TSH, Free T4 & Free T3
From a basic TSH and T4 screen to a full antibody workup, thyroid panels for every level of detail.
View panelThyroid Blood Test Panels: TSH, Free T4, Free T3 & Antibodies
From a basic TSH check to a full antibody workup, a complete menu of thyroid tests and panels, currently 20% off.
View panelComprehensive Blood Test Panel: 17 Tests, 78 Biomarkers
One blood draw, 78 biomarkers, walk into your next appointment with real data.
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