Study finds no link between thyroid function and MCI

Neither clinical nor subclinical hypothyroidism was associated with mild cognitive impairment in a large, population-based study.

Among almost 2,000 elderly subjects, mild cognitive impairment (MCI) was present in 16% of those with normal thyroid function, 17% of those with clinical hypothyroidism, and 18% of those with subclinical hypothyroidism.

The findings questions the usefulness of pursuing thyroid status as a possible contributor to MCI, wrote the department of psychiatry and behavior sciences, University of Texas Medical School, Houston.

“Cognitive decline and thyroid dysfunction are common in the elderly, and widely held view is that hypothyroidism is a reversible risk factor for cognitive impairment, even though several studies  have shown no such association. Our population-based findings also argue against an association and suggest that neither clinical nor subclinical hypothyroidism is a risk factor for MCI. .... This raises questions about the need for routine testing of thyroid function as a part of the diagnostic work-up in patients with MCI.”

The investigators examined thyroid function and cognitive status among 1,904 elderly subjects who were included in the Olmsted County (Minn.) health care records database. Most (1,588; 83%) were cognitively normal; 316 had MCI. Those with MCI were older (82 vs. 80 years), less educated (13 vs. 14 years), and more often carries of the apolipoprotein E4 risk gene (30% vs. 22%).

Medical comorbidities, including hypertension, coronary artery disease, diabetes, stroke, and depression were significantly more common among those with MCI.

Most of the study patients (1,450) had normal thyroid function. Of these, 84% were cognitively normal and 16% had MCI.

Clinical hypothyroidism was present in 313; of these, 83% were cognitively normal, and 17% had MCI. All subjects were taking thyroid replacement therapy. Subclinical hypothyroidism was present in 141; of these, 83% were cognitively normal and 18% had MCI. None of these subjects were getting thyroid replacement therapy.

In an analysis that controlled for age, education, sex, APOE E4 status, depression, and medical comorbidities, MCI was not significantly associated with either clinical or subclinical hypothyroidism. There was no significant interaction between APOE E4 status and hypothyroidism.