Which Thyroid Blood Test Should You Book? TSH vs FT4/FT3 vs Full Panel

For most people, a thyroid blood test that measures TSH, Free T4 and Free T3, often called a thyroid function test, is the most useful private option. It shows whether your thyroid is underactive or overactive and gives more context than TSH alone. Add thyroid antibodies only if you want to look for an autoimmune cause, such as Hashimoto’s thyroiditis or Graves’ disease.

Thyroid symptoms that lead people to test

The thyroid controls how fast your body uses energy, so problems can affect almost every system. Common reasons to test include:

• tiredness that doesn’t improve with rest;
• unexplained weight gain or weight loss;
• feeling cold or hot when others don’t;
• a fast or irregular heartbeat;
• dry skin, thinning hair or constipation;
• changes to periods;
• a family history of thyroid disease.

The three levels of thyroid test

Test	Markers	Use it when
TSH only	TSH	Simple screening
Thyroid function	TSH, Free T4 and Free T3	You have symptoms or want a fuller picture
Advanced thyroid panel	Thyroid function plus a wider set of markers, such as antibodies	Family history, abnormal past results or a suspected autoimmune cause

What each marker tells you

• TSH (thyroid-stimulating hormone) is made by the pituitary gland. It rises when the thyroid is underactive and falls when it’s overactive.
• Free T4 (thyroxine) is the main hormone the thyroid produces.
• Free T3 (triiodothyronine) is the more active form. It’s most useful when an overactive thyroid is suspected.
• Thyroid antibodies show whether the immune system is attacking the thyroid.

Common patterns include:

• high TSH with low Free T4, which suggests an underactive thyroid;
• low TSH with high Free T4 and/or Free T3, which suggests an overactive thyroid;
• a slightly raised TSH with normal Free T4, sometimes called subclinical hypothyroidism, which often needs a repeat test.

Results always need to be read by a clinician alongside your symptoms.

Why your GP may only test TSH

NHS labs often run TSH first because it’s a sensitive screen. Free T4 is usually added only if TSH is abnormal. That works well for screening, but it can leave you with less information if you have ongoing symptoms. A private test that includes Free T4 and Free T3 from the start gives a fuller picture from one sample.

Choosing between Swift’s thyroid tests

Swift offers a standard thyroid function test and the Ultimate Thyroid Health blood test, which covers a wider set of markers. Choose:

• the standard test if you have symptoms and haven’t been tested before, or want to recheck a previous result;
• the wider panel if thyroid disease runs in your family or you’ve had borderline results before.

Check each product page for the full marker list, then book the thyroid function blood test or the wider panel.

Preparing for your test

• You usually don’t need to fast.
• A morning appointment helps when you’re comparing results over time.
• If you take levothyroxine, ask your prescriber whether to take your dose before or after the test. Don’t stop it.
• Biotin, found in some hair, skin and nail supplements, can interfere with some thyroid tests. Tell the clinician if you take it.

Thyroid test or a wider fatigue screen?

If tiredness is your main symptom, the cause may not be your thyroid. Iron, vitamin B12, vitamin D and blood sugar can all play a part. The Tired All the Time blood test (£160) checks thyroid function alongside these markers in one appointment.

Clinic or home collection

Swift takes venous samples at clinics across Essex, plus Bromley and Sunbury, or at home across Essex, Bromley and Surrey. Use the clinic finder to choose a location.

Monitoring an existing thyroid condition

If you already have a thyroid condition, regular testing helps check your treatment. People on levothyroxine are usually tested a few weeks after any dose change, then less often once their level is stable. Testing at the same time of day, with the same timing of your tablet, makes results easier to compare. Always share your results with your GP or specialist, who will decide whether your dose needs adjusting.

Frequently asked questions

What is the best thyroid blood test?

For most people, TSH with Free T4 and Free T3. Add antibodies if an autoimmune cause is suspected.

Why does my GP only test TSH?

TSH is a sensitive first-line screen. Free T4 is often added only if TSH is abnormal.

Do I need to fast for a thyroid test?

Usually not. A morning sample helps keep results consistent.

Should I take levothyroxine before my test?

Follow your prescriber’s advice. Never stop medication for a test.

What does Free T3 add?

It’s mainly useful when an overactive thyroid is suspected and in some more complex cases.

Where can I get a thyroid test near me?

At Swift clinics across Essex, Bromley and Sunbury, or on a home visit.

Book your thyroid test

If you want the right thyroid test first time, book Swift’s thyroid function test and choose a morning appointment.