What it measures
- TSH (thyroid-stimulating hormone)
- Free T4 (FT4)
- Free T3 (FT3)
Who it’s for
- Tiredness, weight gain or feeling cold (signs of an underactive thyroid)
- Anxiety, weight loss, a racing heart or feeling hot (signs of an overactive thyroid)
- Thinning hair, dry skin or irregular periods
- Monitoring thyroid medication
How your thyroid is controlled
Your pituitary gland releases TSH (thyroid-stimulating hormone) to tell your thyroid how hard to work. The thyroid responds by making T4, which your body converts into the more active T3. When the thyroid is underactive, the pituitary pushes harder and TSH goes up; when it’s overactive, TSH falls. Measuring all three shows where the problem lies.
Easy to miss, simple to treat
An underactive thyroid is much more common in women than men and becomes more likely with age, after pregnancy and with other autoimmune conditions. Symptoms creep up slowly, so they’re easy to put down to stress or getting older. Once it’s diagnosed, treatment with a daily tablet is usually simple.
| Result pattern | What it often means |
|---|---|
| TSH high, free T4 low | Underactive thyroid (hypothyroidism) |
| TSH high, free T4 normal | Mildly underactive (subclinical); often rechecked |
| TSH low, free T4 or T3 high | Overactive thyroid (hyperthyroidism) |
| All within range | Thyroid working normally |
How to prepare
- Drink a couple of glasses of water beforehand: it makes the sample quicker and easier to take.
- No fasting needed. Eat and drink as normal.
- Stop biotin (vitamin B7) supplements, often found in hair, skin and nail products, 48 hours before.
- If you take levothyroxine, have your test before your daily dose unless your doctor has told you otherwise.
How it works at our Croydon clinic
Book online
Choose your test and a time that suits you. No GP referral needed.
A 15-minute visit
A trained clinician takes a small blood sample at 169 Shirley Road, Croydon. Tell us if you’re nervous.
Results in 1–3 days
A clear report showing each result against its reference range.
Next steps
Share your report with your GP, who can arrange NHS follow-up if anything needs it.
Questions
Thyroid function or thyroid antibodies?
Thyroid function shows whether your thyroid is working normally. TPO antibodies show whether an autoimmune condition such as Hashimoto’s is the cause. Many people have both, especially if their results are borderline.
I take levothyroxine. When should I test?
Have your blood taken before your daily dose, unless your doctor has said otherwise, and take your tablet straight afterwards.
How much is a Thyroid Function Test in Croydon?
£100 at our clinic at 169 Shirley Road, Croydon, including taking your blood sample and your report.
How quickly will I get my results?
Usually within 1 to 3 days, in a clear report showing each result against its reference range.
Pregnant? See our pregnancy blood tests, including NIPT from £295 and the early gender blood test from 6 weeks.