Is previous hyperthyroidism associated with long-term cognitive dysfunction? A twin study

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 twin pairs discordant for hyperthyroidism, this potential confounding can be minimized. The aim of the study was to examine whether hyperthyroidism is associated with long-term cognitive dysfunction.

Twin case-control study.

Twin pairs discordant for hyperthyroidism were identified by record linkage between The Danish National Patient Registry and 3036 twin pairs from The Danish Twin Registry, who had participated in nationwide surveys on health conditions.

Among other investigations, survey participants had carried out cognitive tests including a Mini-mental state examination (MMSE) and six separate cognitive tests. Based on five of the tests, a composite cognitive score was calculated.

Fifty-five 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 both the intrapair and individual-level analyses, the hyperthyroid twin scored significantly better in the MMSE than did the healthy co-twin (P = 0·023 and P = 0·038, respectively). The same tendency was found in the other cognitive tests, and after analysing twins diagnosed with hyperthyroidism more than 2 years before participating, although none were statistically significant.

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