Endocrine Abstracts

Introduction: Hyperthyroidism has been suggested to adversely affect cognitive function. However, this association could also be caused by genetic and environmental factors affecting both the development of hyperthyroidism and cognitive functioning. By investigating cognitive function within twin pairs discordant for hyperthyroidism, this potential confounding can be minimized. The aim of this study was to examine if hyperthyroidism is associated with long-term cognitive dysfunction.

Methods: Twin pairs discordant for hyperthyroidism were identified by record-linkage between The Danish National Patient Registry, using ICD-8 and ICD-10 codes for hyperthyroidism, and among survey participants of The Danish Twin Registry. Among other investigations, participants had carried out cognitive tests including a Mini-Mental State Exam (MMSE), and six separate cognitive tests. Based on five of the tests a composite cognitive score was calculated. The impact of hyperthyroidism on cognitive function was evaluated by a linear and a conditional logistic regression.

Results: Out of 3036 twin pairs were discordant for hyperthyroidism. The mean time from diagnosis until survey participation was 7.3 years (range: 0–24.1 years). In the inter-pair analysis the hyperthyroid group scored significantly better than the healthy group when controlling for sex, age, zygosity, smoking, and comoborbidities (linear regression: P=0.038). In the intra-pair analysis the hyperthyroid twin scored significantly better in the MMSE than did the healthy co-twin (paired t-test; P=0.023). When stratifying for time since diagnosis in a paired logistic regression, no statistically significant associations for any of the other cognitive tests were found.

Conclusion: Utilizing discordant twin pairs to control for genetic as well as environmental confounding, we could not demonstrate any clinically relevant negative impact of previous hyperthyroidism on long-term cognitive function.