Thyroid dysfunction and dementia. An unlikely relationship?

The thyroid regulates metabolism throughout the body through the release of thyroid hormones. Thyroid function can be influenced by a variety of dietary micronutrients, such as iodine and vitamin B12.

Reduced thyroid function, called hypothyroidism, slows the metabolism, leading to weight gain, while elevated thyroid activity, called hyperthyroidism, speeds up the metabolism, leading to weight loss.

While the effects on body weight are often the most visible, changes to thyroid function impact all metabolically active tissues, including the most energetically demanding organ – the brain.

The brain is highly sensitive to levels of thyroid hormone. Insufficient thyroid hormone during development can result in mental retardation. In adults, clinical hypothyroidism is a cause of cognitive impairment which can be reversible upon the restoration of thyroid hormone levels.

In subclinical hypothyroidism, there are signs of reduced thyroid activity, but thyroid hormone levels are above the threshold for a clinical diagnosis. While obvious effects on cognitive function tend to be absent in individuals with subclinical hypothyroidism, sensitive brain imaging techniques indicate that brain activity is suboptimal.

In healthy individuals, having thyroid hormone levels near the higher end of the normal range has been associated with better performance in specific cognitive domains. Together, this indicates that proper thyroid function is important for optimal cognitive function. Due to conflicting evidence, it has been less clear whether thyroid dysfunction also poses a risk for irreversible forms of cognitive impairment, such as dementia.

Although hypothyroid-related cognitive impairment is considered reversible, the response to thyroid hormone replacement therapy is variable and often incomplete. This is likely related to the way thyroid hormone works in the body.

The thyroid primarily releases thyroid hormone in a low activity form called T4. T4 gets taken up into cells where it is locally converted into a highly active form called T3. Therefore, what matters most is the amount of T3 within a given tissue, like the brain, rather than the circulating level.

With respect to dementia, there is evidence that the levels of T3 are reduced in the Alzheimer’s disease brain, and that some of these genetic variants are associated with dementia risk.

While some studies indicate that hypothyroidism is associated with increased risk for dementia, others find no association, or that thyroid hormone therapy increases risk. This may stem from the way hypothyroidism is measured in different studies.

Since we currently lack a good way to measure brain levels of thyroid hormone, diagnoses are made from blood levels. Dementia risk, then, may be different for people classified as hypothyroid based on blood levels. People who have low thyroid hormone in the blood may still have normal thyroid hormone levels in the brain, and vice versa.

Changes in thyroid function, such as subclinical hypothyroidism, is often seen in aging, but findings from studies do not support the use of thyroid hormone supplementation for this indication. Changes to certain dietary micronutrients also impact thyroid function. Thus, it is better to focus on interventions to improve overall health, such as diet, exercise, and the management of other chronic conditions.

Article submitted by the HPA Group