Requests for free triodothyronine (Free T3, FT3)

In normal circumstances there is no need to request FT3 when assessing thyroid function or monitoring thyroxine therapy – serum FT3 concentration is often normal in hypothyroidism. Measurement of FT3 has no part to play in the diagnosis or monitoring of hypothyroidism and should not be requested when thyroid function tests are being used for this purpose or in a ‘screening’ setting. Serum FT3 is often elevated in Grave’s Disease and other causes of hyperthyroidism. In these settings, TSH is low and the low value triggers the automatic addition of FT3 in the laboratory. Measurement of FT3 is useful in the diagnosis and monitoring of most causes of hyperthyroidism.

Following on from the short article about the appropriate use of the FT3 assay in the previous update, the laboratory ceased to provide a routine service for FT3 from 21st August July 2017. After this date, all requests for FT3 will be reviewed and only processed when there is clinical information on the request to justify the test. This initiative has been discussed with and has the support of the endocrinologists in Te Whatu Ora – Southern. It was also been approved by the Te Whatu Ora Clinical Laboratory Advisory Group.

Normal laboratory practice is to assay TSH first and then add FT4 and /or FT3 as indicated by the TSH
result and clinical details. This practice will continue. Therefore please:

  • Provide clinical details (including relevant therapy) to help in the appropriate ‘reflex addition’ of additional FT4 and FT3 tests
  • Request TSH if ‘screening’ for thyroid disease (FT4 and FT3 will be reflexed according to the TSH result and clinical information provided)
  • Don’t routinely request FT3 when investigating possible thyroid disease as the laboratory will automatically add it when the TSH is clearly low.
  • Remember FT3 has no role in the investigation or monitoring of hypothyroidism and should not be requested in this setting.

For any further queries please contact: Ian Phillips FFSc(RCPA), FRCPath(UK), Clinical Biochemist
T: 03 4702921 – M: 0212294532 – E: