TSH (thyroid-stimulating hormone) is the most commonly used blood test for assessing thyroid function. It’s produced by the pituitary gland and tells your thyroid how much hormone to make. Because of this feedback relationship, TSH is often the first marker doctors check when investigating symptoms like fatigue, weight changes, or feeling unusually cold or hot.
What does TSH actually measure?
TSH doesn’t measure your thyroid hormone directly; it measures the signal your pituitary gland sends to your thyroid. When thyroid hormone levels are low, the pituitary produces more TSH to try to stimulate the thyroid into action. When thyroid hormone levels are high, the pituitary produces less TSH. This means TSH moves in the opposite direction to your actual thyroid hormone levels; a high TSH usually suggests an underactive thyroid, while a low TSH usually suggests an overactive one.
What does a high TSH mean?
A raised TSH often points towards an underactive thyroid (hypothyroidism), where the gland isn’t producing enough hormone and the pituitary is working harder to compensate. Common symptoms include fatigue, weight gain, feeling cold, dry skin, and low mood.
What does a low TSH mean?
A low TSH can suggest an overactive thyroid (hyperthyroidism), where excess hormone is suppressing the pituitary’s signal. Symptoms can include weight loss, a fast heartbeat, feeling unusually hot, anxiety, and difficulty sleeping.
Why is TSH often tested alongside other markers?
TSH alone doesn’t tell the whole story. It’s commonly measured alongside:
- Free T4 (thyroxine): the main hormone the thyroid produces, giving a direct measure of thyroid output.
- Free T3 (triiodothyronine): the more active form of thyroid hormone.
- Thyroid antibodies (TPO and TgAb): can indicate an autoimmune cause, such as Hashimoto’s thyroiditis.
Together, these give a much fuller picture than TSH on its own, particularly useful if your TSH is borderline, or your symptoms don’t fully match your TSH result.
What’s a normal TSH range?
Reference ranges vary slightly between laboratories, but a typical adult range is roughly 0.4–4.0 mIU/L. Your results will always be given alongside the specific laboratory’s reference range, so you can see exactly where you fall.
When should you test your TSH?
Consider testing if you have persistent fatigue, unexplained weight change, feeling unusually cold or hot, hair thinning, low mood, or a family history of thyroid disease. TSH is included in our Thyroid Function Test and our General Health & Wellbeing Screen.
Not sure which test to choose? Use our free Test Finder for a personalised recommendation in under a minute.
Frequently asked questions
Is TSH the only test I need for my thyroid?
TSH is a good starting point, but Free T4, Free T3 and thyroid antibodies give a fuller picture, especially if your result is borderline or symptoms persist.
Can TSH fluctuate day to day?
TSH can vary somewhat, including with the time of day, but significant or persistent abnormalities are generally meaningful and worth discussing with a clinician.
Do I need to fast for a TSH test?
No fasting is generally required for TSH, though we’ll confirm this when you book if it’s part of a wider panel.
My TSH is borderline, should I worry?
Borderline results are common and don’t necessarily mean a problem. They’re often monitored over time or investigated further with additional markers, guided by your GP.
How quickly will I get my results?
Most results are available within 1–2 working days and are reviewed by a GMC-registered doctor before you receive them.
Important note
This article is for general education only and is not medical advice or a diagnosis. Reference ranges vary between laboratories, and any abnormal result should be discussed with your GP or a qualified clinician. Trupoint Health blood tests are analysed by a UKAS-accredited laboratory and reviewed by a GMC-registered doctor.
Medically reviewed by: ____________ (GMC-registered), pending review.
This article is for general education only and is not medical advice or a diagnosis. Persistent or severe symptoms should always be discussed with your GP or a qualified clinician. Every Trupoint Health blood test is analysed by a UKAS-accredited laboratory, and your results are reviewed by a GMC-registered doctor.