Serum leptin, thyroxine and thyroid stimulating hormone levels interact to affect cognitive function among US adults: evidence from a large representative survey

We examined whether serum leptin, T4 and TSH levels are associated with and interact to influence cognitive performance among US adults. Data from the National Health and Nutrition Examination Survey III (1988–94) were used. Measures included a battery of neuropsychological tests and serum leptin, thyroxine and thyroid stimulating hormone (TSH) levels (20–59y: n=1114–2665; 60–90y: n=1365–5519). Among those aged 20–59y, the middle tertile of leptin (second vs. first tertile) was inversely related to the number of errors on the symbol digits substitution test. Increased thyroxine level was associated with a poorer performance on the serial digits test in the 20–59y age group, but better performance on the Math test in the 60–90y group. TSH was associated with poor performance on various tests in the 20–59y group, but better performance in the 60–90y group. Significant antagonistic interactions were found in both age groups between thyroxine and TSH for a number of tests, and between leptin and thyroxine in the 60–90y group in their association with word recall‐trials score. Conducting prospective cohort studies may provide further insight into potential interventions, which in turn would assess interactive effects of various hormonal replacement regimens.