Blood Test - General Points

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Thyroid function tests

Thyroid function tests are blood tests which help to check the function of the thyroid gland. They are mainly used to detect hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid).

What are thyroid function tests used for?

Thyroid function tests are usually done to find out whether the thyroid gland is working properly. This is mainly to diagnose underactive thyroid (hypothyroidism) and overactive thyroid (hyperthyroidism). Thyroid function tests can also be done to:

• Monitor treatment with thyroid replacement medicine for people who have hypothyroidism.
• Check thyroid gland function in people who are being treated for hyperthyroidism.
• Screen newborn babies for inherited problems with the thyroid.

How do thyroid function tests work?

Usually the first test to check thyroid function measures the levels of TSH in your blood. In people with hypothyroidism (underactive thyroid) the amount of TSH will usually be high. This is usually because the thyroid is not making enough T3 to stop the pituitary producing TSH. If the level of TSH is high, you will usually have further tests to check the levels of T3 and T4 in the blood.

In people with hyperthyroidism (overactive thyroid) the level of TSH will usually be low. This is usually because the thyroid gland is making too much of its hormones. When levels of T3 and T4 are high, the pituitary is 'turned off' and the amount of TSH produced is less. If you are found to have low levels of TSH you may have some more blood tests to check the levels of T3 and T4 in the blood. These tests may help doctors to find a specific cause of the low TSH.