The TSH medical abbreviation stands for thyroid stimulating hormone, a messenger made by the pituitary gland at the base of the brain that tells the thyroid how much hormone to produce. A TSH blood test is the most common first check of thyroid function.
It is also called thyrotropin. You will see it on routine lab panels, in notes about tiredness or weight change, and in follow-up for people taking thyroid medicine. The tricky part for many readers is that TSH moves in the opposite direction from thyroid activity, which this page explains along with free T4 and typical ranges.
At a glance
Stands for
Thyroid stimulating hormone (thyrotropin)Made by
The pituitary glandNormal range
Often about 0.4 to 4.5 mIU/L; labs set their own rangeUsual companion test
Free T4 (free thyroxine)Used in
Primary care, endocrinology, prenatal careWhere you see it
Lab panels, endocrine notes, medication follow-up
Why TSH moves opposite to the thyroid
The pituitary and thyroid work as a feedback loop, much like a thermostat. When thyroid hormone in the blood is low, the pituitary raises TSH to push the thyroid harder. When thyroid hormone is high, the pituitary lowers TSH.
- High TSH usually suggests an underactive thyroid, called hypothyroidism.
- Low TSH usually suggests an overactive thyroid, called hyperthyroidism, or too much thyroid medicine.
Less often, the problem sits in the pituitary itself, and then TSH and thyroid hormone can move together. That is one reason doctors often add a free T4.
TSH medical abbreviation on a lab report
Results show the value, the unit and a flag when it falls outside the range. A short line can say a lot.
| Part | Meaning |
|---|---|
| TSH 8.9 mIU/L | Thyroid stimulating hormone measured in milli-international units per liter |
| (H) | Above the lab's reference range |
| Free T4 0.7 | The unbound thyroid hormone, here on the low side |
| Recheck in 6 to 8 weeks | Repeat testing after the hormone has time to settle |
Primary care note: "Fatigue, cold intolerance. TSH elevated, free T4 low, consistent with primary hypothyroidism. Start levothyroxine, recheck TSH." Here the pattern fits a thyroid gland that is not keeping up. The dose in any real note is chosen by the prescriber.
Typical ranges and what shifts them
Many adult labs use a range of about 0.4 to 4.5 mIU/L, but cutoffs vary between labs and change in pregnancy, older age and childhood. A value just outside the range may simply be repeated.
Several things can nudge the number:
- Recent illness or a hospital stay.
- Time of day, since TSH runs a little higher at night.
- Some medicines, including steroids and amiodarone.
- High-dose biotin supplements, which can interfere with some lab methods.
Tell your clinician about supplements before the draw, as they may suggest pausing some.
For students reading endocrine notes
Remember the inverse rule and you will decode most TSH lines. In charting, write the value with units and pair it with free T4 when both were drawn. Results inside the range may be charted as WNL, though many teachers prefer the actual number. When a patient takes thyroid medicine, TSH is usually rechecked several weeks after any dose change, because the level takes time to settle. Screening TSH is also routine in people with type 1 diabetes, since autoimmune thyroid disease is more common in that group.
Questions for your clinician
- Was free T4 checked too, and what did it show?
- Is my result far outside the range or only slightly?
- Could a recent illness, medicine or supplement affect it?
- When should it be checked again?
Thyroid values only make sense beside your symptoms and history. Ask your primary care clinician or endocrinologist how your own TSH should be read.
Frequently asked questions
What does a high TSH mean?
A high TSH usually means the pituitary is pushing a thyroid that is underactive. Doctors often confirm with a free T4 test before deciding on next steps.
What does a low TSH mean?
A low TSH often points to an overactive thyroid or to more thyroid medicine than the body needs. Rarely it reflects a pituitary problem, which other tests can sort out.
Is fasting needed before a TSH draw?
Fasting is usually not required for TSH alone. When glucose or cholesterol is collected at the same visit, you may be asked to fast, so follow the lab's instructions.
Is TSH a thyroid hormone?
Not exactly. TSH is made by the pituitary gland and acts on the thyroid, while the thyroid itself makes T4 and T3. TSH is the signal, and T4 and T3 are the response.