What do they test in the blood for a thyroid test?

The most likely test you will have performed is a blood test measuring the amount of Thyroid stimulating hormone (TSH) in your blood. This is usually used as a screening test for hypo- or hyperthyroidism. If the value is high, you possibly have hypothyroidism, and if low, hyperthyroidism.

After the TSH, your physician may order a T3 and T4 test, which actually measure the amounts of the hormones in your blood. Your physician may have ordered all of these at the same time to save you a needle stick.

These tests, in addition to a TSH, can help a physician tell if you truly have hypo- or hyperthyroidism, or something called "sick euthyroid syndrome" which is a condition where, during an illness, a person becomes relatively hypothyroid, but has normal thyroid hormones and an elevated TSH. This condition is caused by severe illness and will resolve when the illness has run its course.