Thyroid Blood Tests: TSH, T3, T4 and When You Need Them

What Does the Thyroid Do?

The thyroid is a small, butterfly-shaped gland at the front of your neck. Despite its size, it plays a critical role in regulating your metabolism, energy levels, body temperature, heart rate, and mood. It produces two main hormones: thyroxine (T4) and triiodothyronine (T3). The production of these hormones is controlled by thyroid-stimulating hormone (TSH), which is released by the pituitary gland in the brain.

When the thyroid produces too little hormone (hypothyroidism) or too much (hyperthyroidism), a wide range of symptoms can develop. Blood tests are the definitive way to diagnose and monitor these conditions.

TSH (Thyroid-Stimulating Hormone)

TSH is the most important single blood test for assessing thyroid function. It works on a feedback loop: when thyroid hormone levels are low, the pituitary produces more TSH to stimulate the thyroid; when thyroid hormone levels are high, TSH production falls. This means TSH moves in the opposite direction to thyroid hormones.

• Normal range — Typically 0.4 to 4.0 mU/L, though ranges vary slightly between laboratories.
• High TSH — Suggests the thyroid is underactive (hypothyroidism). The pituitary is working harder to stimulate an underperforming thyroid.
• Low TSH — Suggests the thyroid is overactive (hyperthyroidism). The pituitary reduces TSH because there is already too much thyroid hormone.

TSH alone is often sufficient for initial screening and for monitoring patients already on treatment. However, if TSH is abnormal, further tests are required.

Free T4 (Thyroxine)

Free T4 measures the unbound, active form of thyroxine circulating in the blood. Most T4 is bound to proteins and is inactive; the free fraction is what your body can actually use. Free T4 is the test most commonly ordered alongside TSH to confirm a thyroid diagnosis.

• Normal range — Approximately 9 to 25 pmol/L.
• Low Free T4 with high TSH — Confirms primary hypothyroidism.
• High Free T4 with low TSH — Confirms hyperthyroidism.

Free T3 (Triiodothyronine)

T3 is the more biologically active thyroid hormone. Most T3 is produced by the conversion of T4 in the body's tissues rather than directly by the thyroid. Free T3 is not routinely measured but is useful in specific situations:

• When hyperthyroidism is suspected but Free T4 is normal (T3 thyrotoxicosis).
• When monitoring treatment for Graves' disease.
• When patients on levothyroxine continue to have symptoms despite normal TSH and Free T4.

The normal range for Free T3 is approximately 3.5 to 6.5 pmol/L.

Thyroid Antibodies

Thyroid antibody tests help determine whether thyroid dysfunction is caused by an autoimmune condition. The two most commonly tested antibodies are:

• Thyroid peroxidase antibodies (TPO-Ab) — Elevated in approximately 90% of people with Hashimoto's thyroiditis (the most common cause of hypothyroidism) and around 75% of those with Graves' disease.
• Thyroglobulin antibodies (TgAb) — Also associated with autoimmune thyroid disease. Sometimes measured alongside TPO-Ab for a more complete picture.
• TSH receptor antibodies (TRAb) — Specific to Graves' disease. These antibodies stimulate the TSH receptor, causing the thyroid to overproduce hormones.

Hypothyroidism vs Hyperthyroidism

Hypothyroidism (underactive thyroid)is the more common condition, affecting around 2% of the UK population. Symptoms include fatigue, weight gain, cold intolerance, constipation, dry skin, hair thinning, low mood, and brain fog. Hashimoto's thyroiditis is the most frequent cause. Treatment is with daily levothyroxine tablets, and most people require lifelong treatment.

Hyperthyroidism (overactive thyroid)is less common but can be more acutely dangerous. Symptoms include weight loss, anxiety, tremor, heat intolerance, palpitations, diarrhoea, and eye problems (particularly in Graves' disease). Treatment options include carbimazole, radioactive iodine, or surgery.

When Should You Have Thyroid Blood Tests?

Your GP may request thyroid function tests if you have symptoms suggestive of thyroid disease, or as part of routine investigations for fatigue, weight changes, or mood disturbance. Thyroid testing is also recommended in the following situations:

• Family history of thyroid disease or autoimmune conditions.
• Type 1 diabetes, coeliac disease, or other autoimmune conditions.
• Postpartum period (thyroiditis can develop in the months after giving birth).
• Before and during pregnancy if you have a history of thyroid problems.
• After starting amiodarone, lithium, or immunotherapy drugs, all of which can affect thyroid function.

Monitoring Levothyroxine Treatment

If you are taking levothyroxine for hypothyroidism, regular blood tests are essential to ensure you are on the correct dose. NICE recommends the following approach:

• Check TSH 6 to 8 weeks after starting treatment or changing dose.
• Once stable, check TSH annually.
• Have your blood taken before your morning levothyroxine dose — taking it beforehand can temporarily raise Free T4 and give misleading results.
• Aim for a TSH within the lower half of the reference range (typically 0.4 to 2.5 mU/L) for most patients.

Do Thyroid Tests Require Fasting?

Thyroid function tests do not require fasting. You can eat and drink normally before your test. However, consistency is helpful — if you always have your blood taken in the morning before food and before taking levothyroxine, your results will be more comparable over time.

Getting Tested at Home

Thyroid blood tests are among the most commonly requested tests through home phlebotomy services. A qualified phlebotomist can collect your sample at home, typically first thing in the morning before you take your medication, and send it to an accredited laboratory. This is particularly convenient for the annual monitoring that most people on levothyroxine require.

Lola Dispatch connects you with DBS-checked, qualified phlebotomists across the UK. Whether you need a routine thyroid check or a comprehensive panel including antibodies, a home blood test can save you a trip to the surgery and fit around your schedule.