A significant body of science now links poor thyroid function and cognitive decline, including Alzheimer's risk (doubling the risk for women). The information is proving interesting, and not what many would expect. In fact, taking thyroid hormone to help offset aging issues may make cognitive decline worse.
The data shows that individuals with a baseline higher TSH at age 65 had lower scores on mental performance, even though they did not at that time have cognitive impairment. Interestingly, high normal levels of T4 signified even worse cognitive scores and a stronger likelihood of cognitive decline as time goes by. The researchers did not understand why this was the case but correctly warned that taking thyroid medication, as is done in many anti-aging clinics, is likely to make cognitive decline worse.
The reason elevated T4 makes brain function worse is because thyroid function in your brain is regulated by every cell, as different from the rest of your body. In other words, every cell in your brain can change T4 into biologically active T3, whereas your liver is the main site of T3 activation for the rest of the cells in your body.
The reason your brain works this way is because your head has to be able to stay awake in times of extreme famine so that you could hunt if you happened to see something to hunt. However, the rest of your body needs to be in "hibernation mode" so that you don't use too many calories. This is a key survival mechanism.
However, one of the first signs of low thyroid is actually an elevation of T4 - which means it is not getting converted to T3. Such an elevation conks out thyroid activation in your brain, which is why a heavy or sluggish feeling in your head is a key symptom of poor thyroid function. It is now obvious that unless this is corrected it will lead to cognitive decline.
However, taking thyroid hormone may raise blood levels of T4 and could easily make this problem worse. This would especially be the case in a person lacking the key nutrients to convert T4 into T3 or to protect the thyroid gland from oxidative damage while thyroid hormone is being produced.
This data means that how your head feels is one of the most important indicators of successful thyroid support. If you are taking medication and your head isn't feeling better you could actually be making your brain deteriorate faster.
A common assumption is that an elevation of TSH, even when in the normal range, is a sign of sluggish thyroid function. Data coming from a study of centenarians (the very long-lived) shows that there are a few very important caveats to understand. As a person successfully enters very old age the TSH level is actually higher. This is not reflecting the first stages of hypothyroid, it is reflecting a new fitness of the thyroid system in a person who is aging healthfully. This finding throws a major monkey wrench into the simplistic idea of adjusting thyroid hormone levels to meet lab test scores. Taking thyroid hormone would lower TSH in this population, thereby disturbing the thyroid fitness that is associated with their longevity!
My take home message is as follows. How you feel is more important than lab test numbers. Try to fix your thyroid without any medication, by using nutrition to protect your thyroid gland and liver, and by providing the key nutrients that enable the healthy activation of T4 to T3.
If your TSH starts to rise as you get older realize that this could be a first sign of poor thyroid function or a sign of adaptation and thyroid fitness that is associated with longevity. How do you tell the difference? By how your head feels. If your head is mostly alert and cognitively clicking along, then you are most likely in the health zone. If your head is heavy and tired then you need to work on improving thyroid function as a priority, as otherwise you are not on a healthy track but on a path to rapidly deteriorating cognitive function.
The same lab test score means two entirely different things, which can be differentiated based on how you are feeling. The other important point is, do what you can to naturally get your body to work better without taking actual hormone. If you need hormone, try to take the lowest dose you can (of course work with your doctor), while naturally fixing your metabolism.
Thyroid hormone can for some make symptoms feel better for a while. These studies indicate that long term use, especially in older individuals, runs a high risk for accelerated brain aging.