Rationalizing Thyroid Function Testing: Which TSH Cutoffs Are Optimal for Testing Free T4?

Context: Thyroid function testing often uses thyrotropin (TSH) measurement first, followed by reflex testing for free thyroxine (T4) if TSH is outside the reference range. The utility of different TSH cutoffs for reflex testing is unknown.

Objective: To examine different TSH cutoffs 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).

Results: In the clinical cohort, restricting free T4 measurement to individuals with TSH <0.3 or >5.0 mU/L resulted in a 22% reduction in free T4 testing compared with a TSH reference range of 0.4 to 4.0 mU/L; using TSH cutoffs of 0.2 and 6.0 mU/L resulted in a 34% reduction in free T4 testing. In the community cohort, the corresponding effect was less: 3.3% and 4.8% reduction in free T4 testing. In the clinical cohort, using TSH cutoffs of 0.2 and 6.0 mU/L, elevated free T4 would go undetected in 4.2% of individuals with TSH levels of 0.2 to 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 levels of 4 to 6 mU/L; in 94%, free T4 was marginally reduced and unlikely to indicate clinically relevant hypothyroidism.

Conclusions: Setting TSH cutoffs at 0.1 to 0.2 mU/L less than and 1 to 2 mU/L greater than the reference range for reflex testing of free T4 would reduce the need for free T4 testing, with minimal effect on case finding.