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 mu g/L at baseline consistent with moderate iodine deficiency. Hyperthyroidism (thyrotropin < 0.4 mIU/L) occurred in 37 (9.1%) participants. Kaplan-Meier survival curves showed an increase in mortality among participants with hyperthyroidism after IF. There was no significant association between hyperthyroidism and mortality before IF compared to euthyroid participants, but after IF hyperthyroid subjects had an increased mortality (adjusted hazard ratio: 2.22, 95% confidence interval: 1.44-3.44). Conclusions IF was associated with raised mortality among older adults with a history of hyperthyroidism and moderate iodine deficiency. Our results highlight the need for cautious iodine supplementation and for monitoring of IF.

Automated summary

The introduction of iodine fortification of salt in Denmark led to a transient rise in hyperthyroidism, particularly affecting older adults. This study found an association between hyperthyroidism and increased mortality in older adults with moderate iodine deficiency. Results emphasize the importance of cautious iodine supplementation and monitoring of iodine fortification programs.