Your thyroid is a small, butterfly-shaped gland at the base of your neck, but it has an outsized job: it sets the pace of your metabolism. The hormones it makes influence your energy, weight, mood, heart rate, temperature and much more. When it runs too slow or too fast, the effects can be wide-ranging — and a simple blood test is the main way to check how it's working.
What the test actually measures
A standard thyroid blood test looks at two things:
- TSH (thyroid-stimulating hormone) — released by the pituitary gland in your brain to tell the thyroid how hard to work. It's the single most useful thyroid marker.
- Free T4 (thyroxine) — the main hormone the thyroid produces. "Free" means the portion available for your body to use.
The clever part is how they move together. TSH works like a thermostat: if your thyroid is underperforming, the brain pushes out more TSH to try to stimulate it, so a high TSH usually points to an underactive thyroid. If the thyroid is overactive, the brain dials TSH down, so a low TSH suggests an overactive gland. Reading TSH and free T4 together gives the clearest picture.
What an underactive thyroid looks like
An underactive thyroid (hypothyroidism) — typically a high TSH with a low or low-normal free T4 — slows the body down. Common symptoms include:
- Tiredness and low energy
- Weight gain and feeling the cold
- Dry skin, brittle hair and constipation
- Low mood and difficulty concentrating
What an overactive thyroid looks like
An overactive thyroid (hyperthyroidism) — usually a low TSH with a raised free T4 — speeds things up. This can cause weight loss despite a good appetite, a racing or irregular heartbeat, feeling hot and sweaty, anxiety or restlessness, tremor, and looser bowels.
When thyroid antibodies are worth adding
If your basic thyroid bloods are abnormal — or if they're normal but symptoms strongly persist — measuring thyroid antibodies can help. These check for autoimmune thyroid conditions: raised TPO or thyroglobulin antibodies point towards Hashimoto's (the commonest cause of an underactive thyroid), while a different antibody is linked to Graves' disease (a common cause of an overactive thyroid). A comprehensive thyroid panel adds free T3 and antibodies for this fuller assessment.
Check your thyroid
Our thyroid test measures TSH and free T4 — the core check for an underactive or overactive thyroid. If your symptoms need a deeper look, a comprehensive panel adds antibodies and free T3.
Book a thyroid testWhat your results mean
Thyroid results are always interpreted alongside your symptoms, not as numbers on their own. Levels can be affected by pregnancy, some medications and even the time of day, and a "borderline" result is often repeated before any conclusion is drawn. Sometimes TSH sits slightly outside the range while free T4 stays normal — a "subclinical" pattern that your doctor may simply monitor rather than treat straight away.
When to get checked
If you have several of the symptoms above — particularly unexplained tiredness, weight change, feeling unusually hot or cold, or mood changes — a thyroid test is a sensible first step. It's quick, and knowing your numbers lets you and your doctor act on facts rather than guesswork.
Check your thyroid with OneMedicine
Book online with OneMedicine — CQC-registered clinic in Birmingham, or a home visit / local clinic nationwide. Doctor-reviewed results, most within 24 hours.
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Specialist interest: Cardiovascular Medicine · OneMedicine, CQC-registered clinic