Oral Presentation The Joint Annual Scientific Meetings of the Endocrine Society of Australia and the Society for Reproductive Biology 2017

Rationalising thyroid function testing: which TSH cut-offs are optimal for testing free T4? (#155)

Meg Henze 1 , Suzanne Brown 2 , Narelle Hadlow 3 , John Walsh 2 4
  1. Department of Obstetric Medicine, King Edward Memorial Hospital, Subiaco, WA, Australia
  2. Department of Endocrinology and Diabetes, Sir Charles Gairdner Hospital, Nedlands, WA, Australia
  3. Department of Biochemistry, PathWest Laboratory Medicine, Nedlands, Western Australia, Australia
  4. School of Medicine and Pharmacology, University of Western Australia, Perth, Western Australia, Australia

Context: Thyroid function testing is commonly based on measurement of TSH first, then reflex testing for free T4 if TSH is outside reference range limits. The utility of different TSH cut-offs for reflex testing is unknown.

Objective: To examine different TSH cut-offs for reflex testing of free T4.

Design, setting and patients: We analyzed concurrent TSH and free T4 results from 120 403 individuals from a single laboratory in Western Australia (clinical cohort) and 4568 Busselton Health Study participants (community cohort).

Results: In the clinical cohort, restricting measurement of free T4 to individuals with TSH below 0.3 or above 5.0 mU/L resulted in a 22% reduction in free T4 testing compared with TSH reference range limits of 0.4 and 4.0 mU/L; using TSH cut-offs of 0.2 and 6.0 mU/L resulted in a 34% reduction in free T4 testing. In the community cohort, the impact was less: 3.3% and 4.8% reductions in free T4 testing respectively. Using TSH cut-offs of 0.2 and 6.0 mU/L, elevated free T4 would go undetected in 4.2% of individuals with TSH 0.2-0.4 mU/L; in most, free T4 was marginally elevated, and unlikely to indicate overt hyperthyroidism. Low free T4 would go undetected in 2.5% of individuals with TSH 4-6 mU/L; in 94%, free T4 was marginally reduced and unlikely to indicate overt hypothyroidism.

Conclusion: Setting TSH cut-offs 0.1-0.2 mU/L below and 1-2 mU/L above reference range limits for reflex testing of free T4 reduces the need for free T4 testing, with minimal impact on case-finding.