Is there a link between your hormones and the risk of dementia later in life?

Thyroid hormones play a clear role. People with an underactive thyroid have approximately 40% higher odds of developing dementia than people without this condition. Notably, that elevated risk persists even when thyroid levels are normalised through medication (T4). At the same time, one large observational study suggests that people who take the more active hormone T3 alongside T4 have 27 to 31% lower odds of dementia compared with T4 alone. This is not yet a proven causal relationship, but it is a relevant finding for people who are already being treated for an underactive thyroid. A mildly overactive thyroid without obvious symptoms also appears to raise the risk, with an average of 38% higher odds of dementia across nine large follow-up studies involving nearly 50,000 people.

In women, the decline in oestrogen around the menopause may play a role. Women develop Alzheimer's disease twice as often as men, and there are indications that the timing of hormone replacement therapy is crucial. Starting shortly after the menopause appears to be beneficial; starting at an older age or when cognitive symptoms are already present may actually increase the risk. Genetic predisposition and cardiovascular health further complicate this picture. On the basis of the available research, no general recommendation can be made for all women.

In men, a high level of free testosterone (the active, unbound fraction) is associated with considerably lower rates of dementia: 37% lower odds of dementia overall and as much as 51% lower odds of Alzheimer's disease, in a study of more than 186,000 men. Conversely, a high level of the protein that binds sex hormones (SHBG) is associated with a greater risk of dementia, both in men and in postmenopausal women. This protein makes testosterone less available to the brain, which may partly explain the elevated risks. These are observational associations; whether supplementing testosterone also provides protection has not yet been studied.

Stress hormones in depression and adipose-tissue hormones in overweight have also been linked to a higher risk of dementia. Depression in early or middle adulthood more than doubles the risk of dementia later in life. Chronically elevated stress hormones can damage the memory centre of the brain. In overweight, the balance of hormones from adipose tissue is disrupted, but whether those hormones are themselves the cause or are simply tracking alongside other risk factors remains unclear. Finally, diabetes medications such as SGLT2 inhibitors and the GLP-1 agonist dulaglutide show no significant difference in dementia risk relative to one another so far; whether newer agents in that class will do so remains unknown.