Thyroxine or T4,  is one of two major hormones produced by the thyroid gland (the other is called triiodothyronine (T3)). Within the blood, most thyroid hormones are attached to a specific protein, but it is the hormones that are free from these proteins that are able to affect body functions.  This is therefore why we measure both the total and the 'free' thyroxine in your blood sample.