Elevated same-specimen free triiodothyronine despite reference-range free thyroxine after antithyroid prescribing: Two hospital cohorts

Reference-range free thyroxine (FT4) can coexist with elevated free triiodothyronine (FT3) when thyrotropin (TSH) remains low. We quantified this pattern after antithyroid prescribing and assessed sensitivity to incomplete reference metadata METHODS: Two hospital cohorts were analysed separately (A, 2397 adults; B, 2349). Each observation was the first interpretable same-specimen TSH/FT4 pair in days 14-41, 42-83 or 84-168 after the earliest linked prescription. Elevated FT3 was assessed among interpretable results within the low-TSH/reference-range-FT4 profile. A separate scenario applied historical reference configurations to otherwise unclassifiable B results.

Elevated FT3 occurred in 51/194 (26.3%), 39/166 (23.5%) and 37/138 (26.8%) interpretable target specimens at A, and 79/242 (32.6%), 69/257 (26.8%) and 51/212 (24.1%) at B. FT3 interpretability was 99.5-100% at A and 56.9-63.8% at B; all unknown observations at B had recorded FT3 values but lacked reference metadata for classification. Across hospital-window groups, 52.9-69.2% of elevations were no more than 1.2 times their upper reference limit. Unique concordant candidate assignments for 461 B observations yielded scenario estimates of 141/424 (33.3%), 107/403 (26.6%) and 77/345 (22.3%).

Same-specimen FT3 identified additional hormone elevations despite reference-range FT4 after antithyroid prescribing. Most elevations were modest. The conditional finding remained evident under the specified historical-reference scenario. Joint interpretation requires result-specific reference information.