Thyroid Concerns?

But what about my thyroid?

Won’t my thyroid get destroyed on a keto diet?

We have seen this topic come up time and time again. So we figured we might as well take care of it to help everyone who has concerns. You certainly have valid questions, so we want to break down the issue head on.

The main concern is that your thyroid acts as your “metabolism hub.” You might have heard the thyroid is where your metabolism is determined. It dictates how slowly or quickly you blow through energy. What happens to this gland when you drastically change your metabolic pathways, like we do on a ketogenic diet? Things change for sure. In the end, you end up using less of your thyroid than before. We will get into all that.

Like always, how about we start from the beginning – with the physiology of the thyroid. Once we understand that, we can then understand the changes occurring with ketosis.

Thyroid Physiology & Function

The thyroid is a gland in the mid-section of your throat. People consider it the central hub of metabolism because the hormones that come from it stimulate the breakdown of carbohydrates and fats. They control what your DNA decides to code for – both for respiration / metabolism and for other roles in the body. We will focus on its role in metabolism because that is the main change we see with ketosis. There are other roles the thyroid hormones play, but we will only briefly mention them.

Thyroid gland - "metabolism hub"

So when people talk about having a “fast metabolism”, this is what they are referencing – a very active thyroid. Similarly, you break down less food / nutrients when your thyroid is under-active. There are also other implications with over- and under- active thyroids. Because the thyroid is heavily involved in fetal growth, it is important to have a properly active thyroid during pregnancy.

Thyroid hormones: T3 and T4

The “thyroid hormones” involved in cellular respiration are T3 (triiodothyronine) and T4 (thyroxine). The thyroid mainly makes T4, and this converts into T3 in the liver. The active form is T3 – this is the main ingredient for metabolsim.[1] They play important roles in the break down of food. 

They are the little guys that tell your body (DNA) to produce more of the enzymes used in metabolism.[2][3So they are really at the very front-end of this process. Without these messengers, your body would not know what proteins and enzymes to produce for cellular respiration.

When the hormones reach the mitochondria, they bind to receptors. As a result of the hormone stimulating gene expression, certain proteins are coded for metabolism. When this happens, the amount of O2 used for cellular respiration increases.[4][5Thus there is an increase in your basal metabolic rate and heat production.

When your thyroid is producing these hormones, your use of energy goes up. Your body starts working faster to break down nutrients and produce heat.

Other roles

Thyroid hormones are also involved in plenty of other processes in the body.[1][2][4][5] They are heavily involved in fetal and child development. They are also involved in the cardiovascular and central nervous systems. Additionally, they release another hormone called “calcitonin” which helps to lower serum calcium levels. All the hormones help maintain body homeostasis.

These are very important roles, and we will look at these factors when looking at the results / implications of the diet-induced reduction of thyroid hormones. But, as we said before, we are mainly concerned about their role in metabolism.

Thyroid hormone stimulation

In order for your thyroid to produce hormones, your hypothalamus and pituitary gland need to signal to it. When blood levels of the thyroid hormones (T3 and T4) are low, your hypothalamus produces “thyrotropin-releasing hormone” (TRH). This TRH then signals down to the pituitary gland to tell it to produce “thyroid-stimulating hormone” (thyroptropin or TSH).[4] Check out the simplified diagram below.[1]


Stimulating the thyroid (hypothalamus & pituitary gland to the thyroid)

Having T3 and T4 around tell the hypothalamus and the pituitary gland that we don’t need any more thyroid action. Remember, it is these thyroid hormones that impact our metabolism. With more around, our metabolism increases. This means we use produce more ATP (energy), use more O2, and generate more heat.

Summary of thyroid action

Thyroid hormones (T3, T4, and calcitonin) have many functions in the body. It is mainly the active T3 that deals with our body’s metabolism of glucose and fat. (T4 turns into T3 in the liver when needed.) When concentrations of thyroid hormones are “low”, your hypothalamus gets the ball rolling for T3/T4 synthesis. When people get tests done to check their thyroid hormones, it is these T3 and T4 levels that are checked. People are usually concerned with their levels of T3 on a ketogenic diet. Let’s have a look.

Fuel for your thyroid

We keep seeing a lot of people claim that your thyroid needs glucose to function. First of all, your body generates the any glucose you need endogenously through gluconeogenesis. Secondly, where does this idea come from? We check every source that bloggers put out that claims glucose is required. We have yet to find a study that actually claims that as so.

Iodine and tyrosine are required for thyroid hormones – those are its defining factors.[1] But beyond that, we just need to make sure the proper systems are in place for the signaling cascade of your hypothalamus through your pituitary gland, eventually to stimulate the thyroid. Who says there needs to be glucose involved? There is no evidence that your thyroid can’t function on ketone bodies.

The brain usually uses glucose for fuel. It cannot use fatty acids directly. But these are not the only two options – all parts can use ketone bodies. And actually, your brain runs much better on these ketone bodies.They have neuroprotective properties, and your brain uses them very efficiently.[6] So we don’t really see a need for glucose in the stimulation of metabolism. And if there were a need, your body generates its own glucose anyway.


Ketone bodies are fuel for the brain

There is also other clear evidence you don’t need dietary carbohydrate for thyroid function. People on a long-term ketogenic diet would be dropping dead without hormone supplements if the thyroid needed dietary glucose to function. But rather than dropping dead, they are actually doing quite well – better than most. This last point is much more qualitative (“soft” proof), but it still explains the idea that your thyroid can function on a keto diet.

Ketosis and your thyroid

Ketosis is the metabolic state you enter when you restrict your dietary carbohydrates – either through fasting or a high-fat diet. In this context, we are talking about purposeful ketosis through a high-fat diet. Your body is using fatty acids and ketone bodies for fuel (rather than glucose). This has a lot of consequences – but the main concern here is that your levels of T3 will usually drop with a ketogenic diet.[7]

Is this a bad thing? It certainly could be. Hypothyroidism is not very pleasant. But the real question is:

Is decreased serum T3 the same thing as hypothroidism?

We make a distinction between low activity of the thyroid and the disease “hypothyroidism” – which has measurable and negative consequences beyond a low level of T3. We have to look at many factors surrounding thyroid activity. Perhaps the decreased T3 is a benefit of the diet, not a problem with it… Of course, the data published for ketogenic diets and thyroid function are limited. But we can still use the studies that are available that give several hints to a conclusion.

Metabolic rate and ketosis

Basal metabolic rate

There was a very recent study published that shines a great amount of light on metabolism. Being on a ketogenic diet does NOT reduce your resting metabolic rate.[8] This means that although you have less T3 floating around, your body is still burning the same amount of fuel for its respective processes. The researchers also found that body fat decreased significantly, but lean body mass did not. That could be a reason the metabolic rate did not decrease.

The main inference from this study is that being in ketosis does not give you hypothyroidism. It seems as if your body is simply more efficient with the T3. There is no change in your metabolism – just a change in your T3. Who would argue that is a bad thing?

Decreased thyroid activity and longevity

Is having less T3 even a bad thing? There is an interesting study that looked at the genetic factors that contribute to long human lifespans (longevity).[9] They found a common factor among all long-lived individuals: decrease thyroid activity. The longevity had a direct correlation with decreased T3, T4 and TSH. While this study did not specifically look at diet-induced changes in T3, it indicates that having lower T3 (without it being “hypothyroid”) is a good thing. We do not want to infer too much information from this study, but it certainly gives some insight into thyroid function.

We also know that caloric restriction increases longevity.[10][11] Additionally, ketosis mimics the effects of caloric restriction and also leads to longevity.[11] So ketosis leads to longevity, and longevity is associated with lower T3 levels. Maybe the lower T3 levels of ketosis plays a role in helping you live longer… Maybe.

Other factors lower T3 concentrations

First of all, we know that just simply losing weight causes a decrease in thyroid activity and a decrease in conversion of T4 to T3 (the active form).[7] Well, the ketogenic diet certainly causes people to lose weight.

Also, we know that eating less also causes a decrease in T3 levels.[12] Well, the ketogenic diet is almost always accompanied with a decrease in total caloric intake (depending on the person). You just eat less on a high-fat diet. Maybe it is these factors that contribute to a decrease in T3 concentrations.

Is losing weight bad? It also lowers your T3 levels.

Would you say losing weight and eating less are bad? Because they also lower the activity of the thyroid… Also, if people are loosing weight, why would you argue then that their thyroid isn’t functioning actively? As far as we understand, having an under-active thyroid leads to a gain of weight, not a loss…

If people are loosing weight, then clearly their thyroid hormones (and metabolism) are functioning more than adequately. If there are no direct negative signs of hypothyroidism with ketosis, then how is lower T3 bad?

Factors surrounding thyroid action

So, we have less T3 in our system on a keto diet. If your body was truly in need of more T3 for metabolism, then the lower concentrations would stimulate your pituitary gland to generate more TSH. However, the amount of TSH actually decreases slightly on a keto diet.[13] This indicates that your body does not think it needs more T3 – it is doing just fine.

Also, many people claim that your liver is also dysfunctional because not enough T4 is converted into T3.  If we were doing damage on the liver such that less T4 converted into T3 (the active form), then we would see a rise in both T4 and TSH. Your body would signal to the hypothalamus and pituitary gland that you don’t have enough T3. Then you would get more TSH and a build-up of T4 due to the lack of conversion to T3. However, this does not happen. Both TSH and T4 decrease on a ketogenic diet.[13][14]

Thus begs the question: does your body think it needs more T3? It doesn’t appear so… If your body doesn’t think it needs more T3, then it probably doesn’t.

Our conclusions

There are so many benefits to a ketogenic diet that it would be hard to justify avoiding it because of potential lower thyroid activity. That is just not rooted in science or common sense.

So, what the heck is going on then? Ketosis leads to less thyroid activity. But why? Clearly there is evidence this not a bad thing. Perhaps its because your body has better sensitivity to the hormones. If somebody is loosing weight, while also lowering their T3, T4, and TSH, then we know the body is doing just fine. The body can get the same (and even better) jobs done with less T3 than before.

Improvement in T3 sensitivity is awesome!

If anything, we would say that’s an improvement! Why would you want to put more stress on your thyroid to create more hormones when you don’t need them? Also, we have seen that our mitochondria function more efficiently on a diet high in fat. Check it out here.

Maybe then we can extend a hypothesis out: being in ketosis allows your cells to function more efficiently.

Better efficiency

Because another role of the thyroid is thermoregulation (heating of the body)[2][3][4], people will sometimes feel cold on a ketogenic diet. Again, we don’t really see this as a bad thing. You will get over that side effect after a while. Energy being wasted away as heat means that the motors of your cells are not being efficient. If we are producing heat rather than usable work, then we are just spinning our wheels for no reason. We have more than enough movement in our bodies to generate heat, so we don’t need to overdo it by stimulating more thyroid hormones. This might be why having less thyroid hormone can lead to a longer life.

Do not worry about lower thyroid function based on measuring your hormone levels. Only worry if you see the negative consequences of hypothyroidism over a long period of time (such as low energy, sleepiness, etc.). If you feel great, are loosing weight, and have energy (after the keto flu), then there is nothing to worry about. In fact, you should be excited because you have a long life ahead of you!


[1] Brent GA. (2012). Mechanisms of thyroid hormone action. The Journal of Clinical Investigation 122(9): 3035-3043.

[2] “Thyroid gland”. Encyclopedia Britannica. (2018). [Online]. Available: Accessed: 18 May 2018.

[3] Stathatos N. (2012). Thyroid Physiology. Medical Clinics of North America 96(2): 165-173

[4] “The Thyroid Gland”. Open Oregon State – Open Textbook Sites. Anatomy & Physiology. [Online]. Available: Accessed: 18 May 2018.

[5] “Mechanism of Action and Physiologic Effects of Thyroid Hormones”. Colorado State University. Vivo Pathophysiology. [Online]. Available: Accessed: 18 May 2018.

[6] Veech RL. (2004). The therapeutic implications of ketone bodies: the effects of ketone bodies in pathological conditions: ketosis, ketogenic diet, redox states, insulin resistance, and mitochondrial metabolism. Prostaglandins, Leukotrienes and Essential Fatty Acids 70(3): 309-319.

[7] Agnihothri RV, Courville AB, Linderman JD, Smith S, Brychta R, Remaley A, Chen KY, Simchowitz L, Celi FS. (2014). Moderate weight loss is sufficient to affect thyroid hormone homeostasis and inhibit its peripheral conversion. Thyroid 24(1): 19-26.

[8] Gomez-Arbelaez D, Crujeiras AB, Castro AI, et al. (2018). Resting metabolic rate of obese patients under very low calorie ketogenic diet. Nutrition & Metabolism 15: 18.

[9] Rozing MP, Houwing-Duistermaat JJ, Slagboom PE, Beekman M, Frolich M, de Craen AJ, Westendorp RG, van Heemst D. (2010). Familial Longevity Is Associated with Decreased Thyroid Function. The Journal of Clinical Endocrinology & Metabolism 95(11): 4979-4984.

[10] Barja, G., et. al. (2017). The Cell Aging Regulation System (CARS)Reactive Oxygen Species 3(9):148–183.

[11] Veech RL, Bradshaw PC, Clarke K, Curtis W, Pawlosky R, King MT. (2017). Ketone Bodies Mimic the Life Span Extending Properties of Caloric RestrictionInternational Union of Biochemistry and Molecular Biology: Life 69(5): 305-314.

[12] Fontana L, Klein S, Holloszy JO, Premachandra BN. (2006). Effect of Long-Term Calorie Restriction with Adequate Protein and Micronutrients on Thyroid Hormones. The Journal of Clinical Endocrinology & Metabolism 91(8): 3232-3235.

[13] Yancy WS, Foy M, Chalecki AM, Vernon MC, Westman EC. (2005). A low-carbohydrate, ketogenic diet to treat type 2 diabetes. Nutrition & Metabolism 2: 34.

[14] Phinney SD, Horton ES, Sims EA, Hanson JS, Danforth E Jr, LaGrange BM. (1980). Capacity for moderate exercise in obese subjects after adaptation to a hypocaloric, ketogenic diet. The Journal of Clinical Investigation 66(5): 1152-61.

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