Adequate clinical information, including a drug history is essential in order to provide information of most value in the diagnosis and management of thyroid disorders.
The first line investigation is freeT4 and TSH
Further tests, including free T3, will be performed as considered appropriate. Generally free T3 is always elevated if FT4 is elevated and so its measurement is unnecessary.
Free T3 is added to detect T3 toxicosis when FT4 is normal but the TSH is below normal and to monitor T3 toxicosis and to monitor thyroid function in patients on Amiodarone. Otherwise it will not be measured unless there is some other complicating factor that has been discussed with the duty biochemist.
Other assays such as free hormone investigations and TBG are sometimes helpful but not usually necessary. The use of these assays and any interpretative problems can be discussed with the Duty Biochemist.