In this section
It is very important that the correct clinical details are provided with thyroid function requests, including whether the patient is on treatment for hypo or hyper-thyroidism. This enables the correct clinical interpretation to be added to the result.
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 usually unnecessary.
Free T3 is added at clinical authorisation if the results are suggestive of T3 toxicosis. It is alo added if the patient is on treatment for hyperthyroidism, or to monitor some patients taking amiodarone. Otherwise it will not routinely be measured unless there is some other complicating factor that has been discussed with the duty biochemist.
Free T3 is not helpful for patients taking thyroxine for primary hypothyroidism. We are moving to measuring TSH only for such patients, the timeline for any changes will be communicated to users before this improvement is made.
There may occasionally be unusual patterns of results that may indicate interferences in the assays we use. If this is suspected, the sample will be sent away for result confirmation by a different assay. Occasionally, more in-depth investigations may be needed if the results are unusual or do not match the clinical picture. The use of these assays and any interpretative problems can be discussed with the Duty Biochemist.
(Last reviewed August 2024)