Thyroid Function Test (TSH, T3, T4)

No, fasting is not required. If you already take thyroid medicine, it is usually best to give the sample before your morning dose so the reading reflects your baseline.

A high TSH with a low free T4 indicates an underactive thyroid, or hypothyroidism. A mildly raised TSH with a normal T4 is called subclinical hypothyroidism and is not always treated — your doctor decides based on your symptoms and other factors.

Usually the sample is taken before your morning dose, so the reading reflects your baseline rather than the peak just after a tablet. Follow whatever your own doctor has told you, and tell the laboratory when you last took it.

With a blood test. TSH is the main screening test, usually done together with free T4. No fasting is needed. A high TSH with a low free T4 indicates an underactive thyroid. Symptoms such as tiredness, weight gain, feeling cold, dry skin and hair fall may prompt the test, but they overlap with many other conditions, so the diagnosis is made on the blood result rather than on symptoms.

This is the part that catches people out. In everyday speech 'low thyroid' means an underactive one, but a LOW TSH on your report usually points the other way — to an overactive thyroid, or hyperthyroidism. TSH is the signal your brain sends telling the thyroid to work harder. When the thyroid is already producing too much, the brain stops sending the signal, so TSH falls. A HIGH TSH is what suggests an underactive thyroid. Either way the result is read together with free T4, and by a doctor, not from the printed range alone.

From the blood test, not the symptoms — they overlap too much. High TSH with low free T4 means underactive. Low TSH with high free T4 means overactive. Underactive tends to cause tiredness, weight gain, feeling cold, dry skin and constipation; overactive tends to cause weight loss, feeling hot, a racing heart, shakiness and anxiety. But plenty of people have some of both, which is why the test decides it.