Thyroid health and low carb diets
Insulin affects thyroid hormone production and action. This isn’t very surprising, as both hormones have a powerful metabolic role; while insulin signals the body to depress processes that use energy in favor of storing, thyroid’s role is to, at its simplest, regulate the “metabolic choke valve”—increasing or decreasing the rate at which the body’s cells are working. The totality of studies exploring the relationship between insulin and thyroid hormone yield a few important observations.
People that are insulin resistant are usually thyroid resistant. Yes, it’s possible to be resistant to the effects of thyroid hormone. Just like with insulin resistance (where the body is less responsive to insulin and, thus, the body must make more insulin to have an effect), with thyroid resistance, the body must make more thyroid hormone for it to have the desired effect. This is why most individuals usually with high body fat have higher-than-normal thyroid hormone levels, not lower [1,2]. One of the most interesting manifestations of this phenomenon can be found in polycystic ovary syndrome (PCOS). PCOS is a female fertility disorder that arises from insulin resistance—the ovaries begin producing too many male hormones (i.e. androgens) and too few female hormones (i.e. estrogens), which is a direct effect of the high insulin in the blood. These women with PCOS (remember, it’s a disorder of insulin resistance) tend to have larger thyroid glands , and the higher the insulin, the larger the thyroid gland .
Multiple reports show that carbohydrate restriction lowers thyroid hormone levels in the body. In one study, subjects were placed on two low-calorie diets that differed in their carbohydrate content. While both lost weight and had reductions in metabolic rate (which always happens when you eat less), the low-carbohydrate group had the greater drop in thyroid levels . Another study put subjects on two low-carbohydrate diets that replaced the carbohydrates with either protein or fat . Interestingly, but not unexpected, the group that ate the high-fat, low-carbohydrate diet not only had the greater drop in insulin, but also a bigger reduction in thyroid hormone.
Why does thyroid hormone drop more with carbohydrate restriction? Undoubtedly there are multiple reasons, but one simple one: because insulin directly affects the thyroid gland. In particular, the thyroid gland appears to have more insulin receptors in conditions of increased insulin (i.e. type 2 diabetes) . Additionally, insulin drives the growth and proliferation of thyroid cells . Another potentially important connection is that insulin appears to increase thyroid receptors in certain cells throughout the body  (though this likely only happens if the body is sensitive to insulin). This may be another explanation of how improving insulin sensitivity can improve thyroid sensitivity—there are more receptors for thyroid hormone, allowing it to act better. And as thyroid hormone acts better, the body doesn’t need as much of it.
This previous point is worth exploring. Some people get concerned about the evidence that a low-carb diet lower thyroid levels, and even claim that it “wrecks your thyroid”. There is certainly no evidence to support this latter concern. I think it’s helpful to compare thyroid levels to insulin levels when it comes to a low-carb diet. If you were to lose weight with a low-carb diet and see your insulin levels come down during this time, you’d likely be thrilled—it’s a reflection of a healthier, more insulin-sensitive body (i.e., insulin is working better). If you were to lose weight with a similar diet and see your thyroid levels come down, some would want you to think it’s because the diet ruined your thyroid. But why? Based on the lower thyroid levels? Did the low-carb diet ruin your pancreas? Is that why insulin went down? Of course not. Just like with insulin levels, thyroid levels came down because the body is more sensitive to it—thyroid hormone is working better now, so we simply don’t need as much as we did before.
I’m unaware of any condition of human obesity that isn’t in some way linked to insulin—insulin is the master regulator of body fat. This holds true with thyroid disorders. Thyroid hormone affects insulin receptor expression at fat tissue. The most commonly known effect of thyroid hormone is to influence body weight. However, most people that are overweight, as indicated above, have normal to elevated levels of thyroid hormone. However, in those people with genuine hypothyroidism, they will often experience fat gain. This phenomenon, as with all other instances of humans gaining fat, is coupled with insulin. When thyroid levels are low (i.e. hypothyroidism), the amount of insulin receptors on fat tissue goes up by almost double ! In contrast, when thyroid hormone is high (i.e. hyperthyroidism), the amount of insulin receptors on fat tissue drops to less than half. As a reminder, insulin is the signal that tells fat cells to grow or shrink—it’s the key regulator. If there are more receptors, there’s more “growth signal”, whereas fewer insulin receptors would convey a “shrink signal” on the fat cells.
There are no reports, none at all, that a low-carbohydrate diet lowers thyroid hormone too much or in a harmful way. In other words, reducing carbohydrates will not cause hypothyroidism. As carbohydrates are reduced, the lowered glucose in the blood will lead to lower insulin in the blood, not only causing improved insulin sensitivity, but also improved thyroid sensitivity. And if you’re more sensitive to thyroid hormone, you need less of it.
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This article is for informational and educational purposes only. It is not, nor is it intended to be substitute for professional medical advice, diagnosis, or treatment and should never be relied upon for specific medical advice.