Relationship between the Thyroid Function and Cognitive Impairment in the Elderly in Japan

OBJECTIVE: The reference ranges of serum thyroid hormone levels are determined by the values of normal subjects aged 15 or 20 to 60 years old in Japan and may differ from the values in elderly patients. In addition, the relationship between the thyroid function and cognitive function remains controversial. We assessed the thyroid function of elderly subjects ≥60 years old and its impact on the cognitive function in Japanese adults. METHODS: We compared the thyroid function by age group and gender and investigated the effects of cognitive impairment on the thyroid function. This study was a cross-sectional, multi-institutional joint study. PATIENTS: The serum concentrations of thyroid hormones in 1,136 patients were measured; however, those taking thyroid hormones, anti-thyroid drugs, and steroid hormones were excluded. Among them, 1,016 cases in which the cognitive function was evaluated were divided into five groups according to their free thyroxine (FT4) levels. RESULTS: Excluding overt thyroid dysfunction (5.8%), the average age of the 1,070 remaining patients was 77.5 years old. The rate of cognitive impairment was lowest at FT4 levels of 1.1-1.2 ng/dL and highest at FT4 levels ＜0.9 ng/dL for both genders. Thyroid-stimulating hormone (TSH) levels in the elderly varied widely by age group and gender. The upper limit of the reference range of TSH for those ≥60 years old may be higher (7.7-9.2 mIU/L for men; 8.2-8.6 mIU/L for women) than the current range for those ＜60 years old (4.23 mIU/L). CONCLUSION: The thyroid function seemed to be slightly higher (lower TSH and higher FT4) in the population without cognitive impairment than in those with cognitive impairment, except for men in their 90s.