Thyroid dysfunction has thought to possibly contribute to memory disorders, such as Alzheimer’s disease and other types of dementia. Consequently, in patients who are experiencing memory problems, thyroid function tests are commonly ordered. A recent study aimed to see if elderly patients with hypothyroidism were more likely to suffer from mild cognitive impairment than elderly euthyroid individuals (those with normal thyroid function). Mild cognitive impairment is described as a state between normal age-related memory loss and dementia. It usually has minimal, if any, functional impact for the patient. Other studies on the topic have yielded conflicting results.
This study assessed cognitive function by face to face interviews and with detailed neurological and psychological tests. These tests looked at how well people performed in different areas of cognition, including memory, language, visuospatial, attention and executive function. In terms of thyroid function, patients were classified as either euthyroid, clinically hypothyroid or as having subclinical hypothyroidism. Clinical hypothyroidism was determined from medical records. Patients were deemed to have subclinical hypothyroidism if they had a thyroid stimulating hormone level of less than 10mIU/L, a free T4 level of 1.01 to 1.79ng/dL and were not on thyroxine therapy.
From the results of the cognitive testing, 316 of the 1904 participants in the study were found to have mild cognitive impairment, and the remaining 1588 patients were deemed to have normal cognitive function. Mild cognitive impairment was found in 16.3% of euthyroid participants, 17.2% of clinically hypothyroid patients and 17.7% of those with subclinical hypothyroidism. However, after the authors of this study accounted for other factors which were likely to influence the results (age, education, other medical conditions etc.), there was no significant influence of clinical or subclinical hypothyroidism on mild cognitive impairment.
A limitation of this study is that in the patients found to have mild cognitive impairment, there is no indication as to whether the hypothyroidism started before the onset of cognitive impairment or vice-versa. Additionally, the way the study was designed means that a cause-and-effect relationship cannot be determined.
Given the mixed results in other studies, more research is needed to determine the influence of hypothyroidism on cognitive function.
Hypothyroidism and risk of mild cognitive impairment in elderly persons: A population-based study. JAMA Neurology; 71(2):201-207. Parsaik AK, et al. (2014).