Raised mortality in old adults with a history of hyperthyroidism following iodine fortification

Objective A transient rise in the occurrence of hyperthyroidism ensued the introduction of iodine fortification (IF) of salt in Denmark. Older adults are at risk of complications to hyperthyroidism that could prove fatal to vulnerable individuals. We evaluated the association between thyroid function and mortality in older adults before and after nationwide implementation of IF. Design Retrospective cohort study. Patients All 68‐year‐olds from the general population in the city of Randers were invited to participate in a clinical study in 1988 and followed until death, emigration or end of study (31 December 2017) using Danish registries. Measurements Baseline measures comprised of a questionnaire, physical examination and blood and urine samples. Kaplan–Meier survival curves and Cox regression were used to determine the association between thyroid function and death before and after IF. Time‐stratification of results before and after IF was employed due to violation of proportional hazards assumptions in Cox regression. Results Median urinary iodine concentration was 42 µg/L at baseline consistent with moderate iodine deficiency. Hyperthyroidism (thyrotropin