Age and gender adjusted FT3 levels as novel predictors of survival.

CONTEXT: Lower levels of free tri-iodothyronine occur during acute illness, as part of "euthyroid sick syndrome" (ETS). A chronic form of this syndrome also exists.

OBJECTIVE: To determine whether thyroid hormone levels predict long-term survival.

DESIGN & SETTING: This was a "big-data" study of thyroid function tests from samples taken between 2008-2014. Data were crossed with electronic health records (EHRs) for morbidity and mortality. Test results were converted to Age and Gender Adjusted Percentiles (AGAPs). The hazard ratio for death was crossed with ranges of initial AGAPs and change in AGAPs for two subgroups: "not healthy" - subjects with at least one of five chronic conditions registered in their electronic health chart; "healthy" - all others.

PARTICIPANTS: 2,453,091 sets of thyroid function tests from 365,965 distinct patients were evaluated. 258,695 sets remained after excluding patients registered as taking thyroid preparations or anti-thyroid drugs.

MAIN OUTCOME MEASURE: Hazard ratio for death, planned before data collection.

RESULTS: The cohort included 151,868 "not healthy" and 106,827 "healthy" people. After a median of 6.8 years, 5,865/151,868 (10.4%) of the "not healthy" had died and 2,504/106,827 (2.3%) of "healthy" participants. Low initial FT3 AGAPs were predictive of poor survival. The Hazard Ratio (HR) for survival compared between the lowest 5 and highest 50 percentiles of initial FT3 AGAPs for "not healthy" participants was 5.71 (CI - 5.23 to 6.26, p < 0.001), and for "healthy" - 3.92 (CI - 3.06 to 5.02, p < 0.001)).

CONCLUSION: Low FT3 AGAPs predicted poor survival, most strongly among "not healthy" people.