Thyroid
TSH blood test: what a high or low result means
What a TSH (thyroid-stimulating hormone) result means, what high or low TSH can mean, and what to ask your doctor.
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Explain my results → Available on web, iOS & Android.What is TSH?
TSH, or thyroid-stimulating hormone, is made by the pituitary gland in your brain. It tells your thyroid gland in your neck how much thyroid hormone to make. TSH is usually the first test used to check thyroid function. It works like a thermostat: when thyroid hormone is low, TSH goes up to push the thyroid harder, and when thyroid hormone is high, TSH goes down. That means a high TSH usually points to an underactive thyroid, and a low TSH to an overactive one.
Why is this test done?
- To investigate tiredness, weight change, feeling cold or hot, a fast heartbeat or low mood
- To monitor thyroid medicine, such as levothyroxine
- In pregnancy or when planning pregnancy, in some situations
- To check people taking medicines that can affect the thyroid, such as amiodarone or lithium
What does a high TSH mean?
A high TSH usually means the thyroid is underactive (hypothyroidism) and the pituitary is working harder to stimulate it.
Possible reasons include:
- An underactive thyroid, most often caused by the autoimmune condition Hashimoto's thyroiditis
- Too low a dose of thyroid medicine, or not taking it regularly
- Recovering from an illness
- Some medicines, such as amiodarone and lithium
- Mild, borderline rises (subclinical hypothyroidism) that may only need monitoring
What does a low TSH mean?
A low TSH usually means the thyroid is overactive (hyperthyroidism), or that the dose of thyroid medicine is too high.
Possible reasons include:
- An overactive thyroid, for example from Graves' disease or thyroid nodules
- Too high a dose of thyroid medicine
- Early pregnancy
- Serious illness
- Rarely, a problem with the pituitary gland
Why we don't list a "normal range". Reference ranges differ between laboratories and can depend on your age, sex and whether you are pregnant. The range printed next to your result on your own report is the one that applies to you, so use that, and talk to your doctor about what your result means for you.
What can affect your TSH result?
- Thyroid medicine, especially the timing of your last dose
- Biotin supplements, which can interfere with some lab tests
- Recent illness
- Pregnancy
- Time of day, as TSH is slightly higher at night
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Try it with your own results → Paste, upload or photograph your results.Questions to ask your doctor
- Does my TSH mean my thyroid is underactive or overactive?
- Do I need a free T4 or thyroid antibody test?
- If I take levothyroxine, does my dose need changing?
- When should I be retested?
Frequently asked questions
Why does a high TSH mean an underactive thyroid?
TSH is the signal from your brain telling the thyroid to work. When the thyroid is not making enough hormone, the brain turns the signal up, so TSH rises. It is the opposite of what many people expect.
Should I take my levothyroxine before a blood test?
Follow your doctor's advice. Many people are asked to take it as usual, but some clinics prefer the test before your daily dose. Be consistent so results can be compared.
Can biotin affect thyroid results?
Yes. High-dose biotin, found in some hair, skin and nail supplements, can interfere with some thyroid tests. Tell your doctor if you take it; you may be asked to stop for a few days before the test.
Can Patiently AI explain my TSH result?
Yes. Paste your results, or upload a photo, PDF or Word file, and Patiently AI explains them in plain English, sticking to the numbers and ranges on your own report rather than adding its own, and suggests questions to ask your doctor. It is free, needs no account, and removes identifying details on your device first.