A rational approach to "in vitro" thyroid function testing.

A system of "in vitro" thyroid function testing is proposed whereby laboratory staff select the most appropriate screening test depending on the information supplied by the clinician. Total serum triiodothyronine (T3) or thyroxine (T4) are used for screening as appropriate. In borderline cases, secondary tests are performed automatically according to a flow chart. This system improves efficiency and is cost effective, saving approximately 1,800 pounds annually in a laboratory handling about 5,000 requests for thyroid function tests each year.