I was inspired to read up on the thyroid system after a post in the CR Society archives about thyroid singling and longevity.
The Basics of the Thyroid System
The thyroid is a butterfly shaped glad at the base of a person’s neck that is a part of endocrine system, which is a system responsible for the body’s hormone production. Other glands in this system are the pituitary gland, pancreas, ovaries, testes, and adrenal glands; the hypothalamus is considered a neuroendocrine organ. The thyroid produces hormones (thyroxine (T4), triiodothyronine (T3), and calcitonin) which regulate metabolism and affect the growth and functioning rate of other body systems.Other important hormones to this system are TSH and rT3. The thyroid stimulating hormone (TSH) is secreted by the pituitary gland and stimulates the thyroid to secrete T3 and T4. Reverse T3 (rT3) is an isomer of T3 (same molecular structure, but the atoms are arranges differently), and is derived from T4. rT3 does not stimulate the thyroid but it does bind to the T3 receptors, which can block the action of T3.
Hypo/Hyperthyroidism
These two common diseases with the thyroid are often associated with a person’s weight, although there are many other symptoms of Hypo/Hyperthyroidism. Hypothyroidism (usually caused by iodine deficiency, postpartum thyroiditis, autoimmune disease -Hashimoto’s thyroiditis, or ) is often correlated to weight gain (or difficulty losing weight) since reduced thyroid hormones causes the metabolism to slow. On the other hand is Hyperthyroidism, which is the overproduction of thyroid hormones, and is often correlated to a difficulty maintaining weight and increased hunger. **There are a multitude of other symptoms, so read the wiki’s for more info**
Euthyroid Sick Syndrome
Euthyroid sick syndrome occurs when the T3 and/or T4 levels are unusual, but the thyroid gland is not dysfunctional. The condition is often seen in fasting, starvation, and serious illness.
CR and Thyroid Production
The post brought to my attention the relationship between CR and thyroid hormone levels. The post basically points out observation of low thyroid signaling in cases of extreme longevity (such as in certain genetically altered strands of mice, and also in some long lived human families ) is similar to that of CR’ed mammals (rodents, nonhuman primates, and humans) which exhibit functionally similar patterns of thyroid hormone levels: high TSH, normal to low-normal T4, and very low bioactive T3. The low T3 would correlate to the change in a CR’ed person’s weight set point, the weight at which they would not continue to lose weight on a calorie restricted diet. Please check out the original post for full context, and references.
In light of this, I decided to see what I could dig up on PubMed, so here are some studies I found interesting. The first 3 studies below basically confirm the change in thyroid signaling with a CR diet, then I list a couple the studies referenced in the post I mentioned correlating longevity to thyroid hormone levels.
The purpose of this study by Fontana L, et al. was to access the relationship between long-term CR with adequate protein and nutrients and thyroid function. Thyroid function was measured for three groups: calorie restricted practitioners, sex matched sedentary people on a western diet, and body fat matched exercising people on a western diet. Serum T3 in the CR participants was lower, whereas T4, reverse T3, and TSH levels were similar among all groups.
NOTE: The diets of the CR’d people consisted of 1779 ± 355 kcal/d and approximately 23P-49C-28F, the diets of the exercise group consisted of 2811 ± 711 kcal/d and approximately 15P-53C-32F, and the diets of the sedentary group consisted of 2433 ± 502 kcal/d and approximately 17P-52C-31F.
(Abstract Only) This is a paper from Walford, RL, et al. consisting of data collected during biosphere 2. The main observation (of the 50 variables measured) I want to point out here is the participants’ T3 levels decreased by 19%. No other values or observations in relation to this topic were discussed in the abstract, but I thought this was worth noting.
NOTE: I would like to see the full study to compare all measured thyroid hormone levels with the Fontata group (the first study I presented). Why? Well, the macro percentages in their diets were quite different among the two studies. The biosphere 2 participants diet consisted of 1750-2100 kcal/d and approximately 12P-77C-11F. This was much lower protein and fat with higher carbs than the Fontana group.
(Abstract Only) The levels of T3, T4, and TSH were measured in ad lib and CR’ed rhesus monkeys as well as in young rhesus monkeys during the 30% CR adaptation phase. In both CR groups, T3 was lowered with the effect being greater in the older monkeys; neither group showed change in free or total T4. The TSH levels decreased with age except in the long term CR’ed monkeys, in which it increased.
(Abstract Only) Previous studies have reported life extension effects associated with low thyroid hormone levels in animal models, so these researchers sought to see if this correlation extended to humans. Thyroid hormone function was measured in a group of middle-aged children of nonagenarian siblings, with their partners serving as the control group. The researchers observed the test group showed a trend toward higher TSH, lower free T4, and lower free T3 levels when compared to their partners.
(Abstract Only) This study found a positive correlation between high TSH levels and extreme longevity.
