| Marker | Gabriel optimal | Typical lab range | Functional range |
|---|---|---|---|
| TSH | 1 to 2 uIU/mL | 0.45 to 4.5 | 0.5 to 2 |
How TSH works
Your pituitary gland watches how much thyroid hormone is in your blood. When thyroid hormone runs low, it makes more TSH to push the thyroid harder. When thyroid hormone runs high, it makes little or no TSH. That is why TSH moves in the opposite direction to your thyroid hormone.
TSH changes through the day, so many labs prefer an early morning draw. Gabriel's TSH lab guide covers the marker in more depth.
Reading TSH with free T4
Free T4 is the active thyroid hormone your thyroid releases. Read together, the two point in a direction. They don't make a diagnosis on their own.
| TSH | Free T4 | What it can point to |
|---|---|---|
| High | Low | An underactive thyroid (hypothyroidism), often from Hashimoto disease |
| High | Normal | A mildly underactive thyroid, or a temporary change; often rechecked |
| Low | High | An overactive thyroid (hyperthyroidism), for example Graves disease or nodules, or too much thyroid hormone medicine |
| Low | Normal | A mildly overactive thyroid, recent illness, some medicines, or biotin interference |
| Low or normal | Low | Rarely, a pituitary problem |
Thyroid antibody tests (such as anti-TPO) look for autoimmune causes like Hashimoto disease or Graves disease. A comprehensive thyroid panel bundles these markers.
What can skew a TSH result
- Biotin (vitamin B7). High-dose biotin, common in hair, skin and nail supplements, can make TSH read falsely low and T4 and T3 falsely high, which can look like an overactive thyroid. The American Thyroid Association advises stopping biotin for at least 2 days before thyroid blood work. Check multivitamin labels too.
- Medicines. Steroids, dopamine, opioids, amiodarone and lithium can change TSH. Don't stop any medicine without your prescriber's advice.
- Iodine, including iodine contrast dye from a recent CT scan.
- Serious illness. Being very sick can lower TSH for a while even with a healthy thyroid.
- Pregnancy. TSH is often a little low in the first trimester, and pregnancy uses different ranges.
- Age. TSH tends to run higher in older adults, and some labs use a higher upper limit for them.
What to do with an abnormal TSH
- Ask for free T4 if it wasn't run, and antibody tests if the pattern suggests an autoimmune cause.
- Mention biotin and every supplement. If you took biotin, a repeat test after stopping may clear it up.
- Repeat a mildly abnormal result before acting. A single borderline TSH, especially after an illness, often settles.
- If you're pregnant or planning pregnancy, tell your clinician about any thyroid history so they use pregnancy ranges.
Gabriel's optimal band (1 to 2 mIU/L) is narrower than most lab ranges. A TSH between 2 and 4.5 is not a disease; it is a reason to look at symptoms, antibodies and the trend over time.
Get care promptly for a racing or irregular heartbeat, chest pain, severe shaking or anxiety, or unexplained weight loss with a low TSH, or for marked confusion, extreme sleepiness or a very slow heart rate with a high TSH.
Common questions
What is a normal TSH level?
Most labs use roughly 0.4 to 4.5 or 4.8 mIU/L for adults; ranges vary by lab, and some use a higher upper limit for older adults. Pregnancy has its own ranges.
Does a high TSH mean hypothyroidism?
Usually it means your thyroid is underactive, but one result isn't a diagnosis. Your clinician checks free T4 and often repeats the test before deciding.
Can biotin affect a TSH test?
Yes. Biotin can make TSH read falsely low and T4 falsely high. The American Thyroid Association advises stopping biotin at least 2 days before thyroid testing.
Do I need to fast for a TSH test?
No special preparation is usually needed. An early morning draw is often preferred because TSH changes through the day.
What is the difference between TSH and T4?
TSH is the pituitary's signal to the thyroid. T4 is the hormone the thyroid makes in response. When the thyroid is underactive, T4 falls and TSH rises.
Sources
- TSH (Thyroid-stimulating hormone) Test MedlinePlus, U.S. National Library of Medicine
- TSH test MedlinePlus, U.S. National Library of Medicine
- Biotin interference with thyroid function tests (Clinical Thyroidology for the Public, December 2018) American Thyroid Association
- Biotin Interference with Troponin Lab Tests U.S. Food and Drug Administration
This guide is general information, not medical advice. It does not replace a visit with a licensed clinician. Gabriel membership is not health insurance.