What does a thyroid function test check?

A thyroid function test measures TSH, and sometimes free T4 and free T3, to check whether your thyroid is underactive, overactive, or working normally.

What it measures

TSH is usually checked first. Depending on the result, the laboratory may add free T4 and sometimes free T3, the active thyroid hormones. The pattern across these markers shows how your thyroid is functioning.

When it is ordered

Symptoms of an underactive thyroid include tiredness, weight gain, feeling cold and low mood. An overactive thyroid can cause weight loss, a racing heart, anxiety and heat intolerance. Testing is also common when planning pregnancy or monitoring a known thyroid condition.

How to prepare

Most thyroid tests do not need fasting. If you take thyroid medication or biotin supplements, mention this, as timing can affect the reading.

Read our thyroid function test guide, or book an appointment.

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