Thyroid blood tests

The primary screening test for thyroid function. TSH is produced by the pituitary gland and regulates the production of thyroid hormones.

Measures the level of the active thyroid hormone thyroxine circulating in your blood and helps assess thyroid function.

May be included if an overactive thyroid is suspected, as it measures the active form of the thyroid hormone triiodothyronine.

Detects antibodies associated with autoimmune thyroid conditions such as Hashimoto's thyroiditis.

May be measured if autoimmune thyroid disease is suspected.

Can help diagnose Graves' disease, a common cause of hyperthyroidism.

No. You do not usually need to fast before a thyroid blood test. You can normally eat and drink as usual unless your appointment includes other blood tests that require fasting. If any preparation is needed, you will be advised before your test.

Thyroid blood test results are usually available within 3-5 days. Some specialist antibody tests may take slightly longer. Your clinician will advise when your results will be available and explain what they mean in relation to your symptoms and medical history.

Thyroid Stimulating Hormone (TSH) is a hormone produced by the pituitary gland in the brain. It controls how much thyroid hormone the thyroid gland produces.

A high TSH level may suggest that the thyroid is not producing enough hormones, which can be associated with an underactive thyroid (hypothyroidism). A low TSH level may indicate that the thyroid is producing too much hormone, which can occur with an overactive thyroid (hyperthyroidism). TSH results are interpreted alongside other thyroid hormone levels.

Free T4 (Free Thyroxine) is one of the main hormones produced by the thyroid gland. It plays an important role in regulating metabolism, energy levels and many body functions.

Measuring Free T4 helps assess how much active thyroid hormone is available in the bloodstream and is usually interpreted alongside TSH to help identify thyroid disorders.

Free T3 (Free Triiodothyronine) is the active form of thyroid hormone that helps regulate the body's metabolism and energy use. It is usually not required for every thyroid assessment but may be measured when an overactive thyroid is suspected or when further information about thyroid function is needed.

A thyroid blood test can help support the diagnosis of Hashimoto's disease but cannot diagnose it on its own. Doctors usually consider a combination of thyroid hormone levels, thyroid antibodies (particularly TPO antibodies), symptoms and medical history.

Hashimoto's disease occurs when the immune system attacks the thyroid gland, gradually reducing its ability to produce thyroid hormones.

Yes, thyroid blood tests can help diagnose Graves' disease when combined with symptoms and clinical assessment. Blood tests may show abnormal thyroid hormone levels and detect TSH receptor antibodies (TRAb), which are associated with Graves' disease.

Graves' disease causes the immune system to overstimulate the thyroid gland, resulting in excessive production of thyroid hormones.

Yes. An underactive thyroid (hypothyroidism) can contribute to weight gain because reduced thyroid hormone levels can slow the body's metabolism. Other symptoms may include tiredness, feeling cold, dry skin and constipation.

However, weight changes can have many causes, so thyroid testing helps determine whether thyroid hormone imbalance may be contributing to changes in weight.

Yes. Both an underactive and overactive thyroid can contribute to hair thinning or hair loss. Thyroid hormones play an important role in regulating the hair growth cycle, and an imbalance may cause changes in hair texture, thickness or shedding.

Hair loss can also be caused by other factors, including low iron levels, stress, hormonal changes and certain medical conditions, so further assessment may be needed.

Yes. Thyroid hormone imbalances can affect fertility in both women and men. In women, untreated thyroid conditions may affect menstrual cycles, ovulation and the ability to conceive. Thyroid function is also important during pregnancy to support the health of both mother and baby.

If you are trying to conceive, are pregnant or planning a pregnancy, your clinician may recommend thyroid testing if there are concerns about thyroid function.

How often thyroid levels should be checked depends on your individual circumstances. People with a diagnosed thyroid condition, those taking thyroid medication or those with ongoing symptoms may require regular monitoring.

If you do not have a thyroid condition, testing frequency will depend on your symptoms, risk factors and medical advice.

Yes. Thyroid blood tests are commonly used to monitor people taking levothyroxine and help determine whether the dose is appropriate. Your clinician may advise on the best timing for your blood test in relation to taking your medication.

Always follow your healthcare provider's instructions and do not stop or change your medication unless advised.

If your thyroid results are abnormal, your clinician will review your results alongside your symptoms, medical history and any relevant risk factors.

Depending on the findings, they may recommend:

• Further blood tests, such as thyroid antibody testing.

• Repeat testing to monitor changes over time.

• Lifestyle advice or treatment.

• Referral to an endocrinologist or specialist if needed.

Abnormal thyroid results do not always mean there is a serious problem, but they should be assessed to ensure you receive the right advice and care.