What is it?
Thyroxine (T4) is one of the two hormones produced by the thyroid gland that help to regulate metabolism. The other hormone is Triiodothyronine (T3).
They should be looked at together, but T4 in the blood can either be "unbound (free)"" or "bound" to proteins.
Unbound T4 is what regulates metabolism, giving a good overall marker for thyroid activity. Too much or too little T4 can be a sign of thyroid disease.
T3, T4 and TSH results are frequently used together for the diagnostics of thyroid problems.