FT4

You do not need to do anything special before this test. 

However, some medicines can affect your results. Tell your doctor about everything you are taking, including prescribed medicines, over-the-counter medicines, and supplements. 

What the test measures 

Thyroxine (T4) is one of two main hormones made by the thyroid gland. The thyroid gland sits in your neck, in front of your windpipe. T4 makes up nearly all of what we call thyroid hormone. The other hormone, triiodothyronine (T3), makes up less than 10%. 

Most T4 in your blood is attached to a protein and cannot act directly on the body. Less than 1% is unattached — this is the free thyroxine (FT4). It is this free portion that the body can actually use. This is why the FT4 test is preferred to a total T4 test. 

Thyroid hormones control your metabolism — the process by which your body converts food into energy. They affect your heart rate, body temperature, mood, weight, and many other functions. 

What your results mean 

If your FT4 is high 

A high FT4 level, alongside a low TSH, usually suggests an overactive thyroid (hyperthyroidism). This means your thyroid is producing too much hormone. 

Common causes of a high FT4 include: 

• Graves’ disease — an autoimmune condition and the most common cause of hyperthyroidism 
• Toxic nodular goitre — overactive nodules within the thyroid gland that produce excess hormone independently 
• Thyroiditis — inflammation of the thyroid that can cause a temporary release of excess hormones 
• Taking too much thyroid hormone medication (levothyroxine) 

If your FT4 is low 

A low FT4 level, alongside a high TSH, usually suggests an underactive thyroid (hypothyroidism). This means your thyroid is not making enough hormone. 

Common causes of a low FT4 include: 

• Hashimoto thyroiditis — an autoimmune condition that gradually damages the thyroid gland, and the most common cause of hypothyroidism in the UK 
• Previous treatment for hyperthyroidism, including radioiodine therapy or surgery 
• Certain medicines, such as lithium and amiodarone 
• Iodine deficiency — uncommon in the UK 
• Problems with the pituitary gland (secondary hypothyroidism), which can result in low TSH alongside low FT4 

FT4 and pregnancy 

Total T4 levels rise during pregnancy because of changes in protein levels in the blood. However, free T4 levels do not usually change significantly in pregnancy. A rise in total T4 does not mean you have thyroid disease. Any changes to total T4 usually resolve after delivery. 

If you are pregnant and being monitored for thyroid problems, your doctor will use FT4 and TSH together, with reference ranges specific to pregnancy.

If your FT4 is abnormal, your doctor will consider the result alongside your TSH and any symptoms you have. They may request additional investigations, such as thyroid antibodies or a scan, to help identify the cause. 

If hypothyroidism is confirmed, treatment is usually with levothyroxine tablets. Most people do well on this treatment, and your dose will be adjusted based on your symptoms and follow-up blood tests. 

If hyperthyroidism is confirmed, treatment options include tablets that reduce hormone production, radioiodine treatment, or surgery. Your doctor will work with you to decide which approach is most suitable. 

Several things can affect your FT4 result or how it is interpreted: 

• Some medicines can interfere with thyroid hormone levels or with the FT4 test itself. These include certain epilepsy medicines, large doses of aspirin, amiodarone, lithium, and some contraceptive pills. FT4 is generally less affected by these than total T4, which is one reason it is preferred. 
• Pregnancy raises total T4, but free T4 is usually less affected. Separate reference ranges may apply in pregnancy. 
• Serious illness can affect thyroid hormone levels, even when there is no underlying thyroid condition. This is sometimes called non-thyroidal illness. 
• Biotin (vitamin B7) supplements taken in high doses can interfere with some FT4 laboratory tests. Tell your doctor or the laboratory if you take biotin supplements. 

Always tell your GP or specialist about all medicines, vitamins, and supplements you are taking before your test. 

FT4 is almost always used alongside other thyroid tests. Your doctor may also request: 

• TSH (thyroid-stimulating hormone) — the first-line test for checking thyroid function 
• FT3 (free triiodothyronine) — particularly useful when hyperthyroidism is suspected but FT4 is normal 
• Thyroid antibodies — to check for autoimmune thyroid conditions such as Hashimoto thyroiditis or Graves’ disease 

The thyroid gland produces hormones that control many of the body’s functions. When it makes too much or too little hormone, it can affect your energy, weight, mood, and heart rate. To learn more, visit the NHS pages on overactive thyroid and underactive thyroid.