Hands up - volunteers to be in the "minimal impact" group who are missed.
Hands up - those who believe that an FT4 test only has any relevance at that instant and does not contribute to the longer-term understanding of patients.
Hands up - those who believe the only real reason is saving money.
J Clin Endocrinol Metab. 2017 Aug 30. doi: 10.1210/jc.2017-01322. [Epub ahead of print]
Rationalizing thyroid function testing: which TSH cut-offs are optimal for testing free T4?
Henze M1, Brown SJ1, Hadlow NC1, Walsh JP1.
1 King Edward Memorial Hospital, Subiaco, Western Australia 6008, Australia; Department of Clinical Biochemistry, PathWest Laboratory Medicine, Queen Elizabeth II Medical Centre, Nedlands, Western Australia 6009, Australia; School of Medicine and Pharmacology, The University of Western Australia, Crawley, Western Australia 6009, Australia; Department of Endocrinology and Diabetes, Sir Charles Gairdner Hospital, Nedlands Western Australia 6009, Australia.
Thyroid function testing often utilizes TSH measurement 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.
To examine different TSH cut-offs for reflex free T4 testing.
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).
In the clinical cohort, restricting free T4 measurement 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. In the clinical cohort, 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 clinically relevant 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 clinically relevant hypothyroidism.
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.
Of course, as in almost every paper which mentions TSH testing, time of day is 100% ignored.
Also, in this abstract, it appears that there is absolutely no possibility of FT4 being out of range if TSH is within range. Maybe the full paper explains more?