No fasting required.

This test measures the amount of free triiodothyronine, or FT3, in your blood. T3 is one of two major hormones produced by the thyroid gland (the other hormone is called thyroxine, or T4). T3 makes up less than 10 per cent of what we call thyroid hormone, while T4 makes up the rest. T3, however, is about four times as strong as T4, and is thought to cause most, if not all, the effects of thyroid hormones.