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4 Reasons Not to Keto with Hypothyroidism

The keto diet seems very attractive. On keto, most people can lose a lot of weight, quickly.

But what about keto and hypothyroidism? As a registered dietitian, I have some big concerns. Continue reading for my thoughts on why the ketogenic diet is bad news for hypothyroidism. 

What is hypothyroidism?

Hypothyroidism occurs when our thyroid gland can’t make enough thyroid hormone to meet its needs. It can also happen when the body’s ability to activate thyroid hormone is compromised. 

The thyroid gland, which sits at the base of your neck, makes different kinds of thyroid hormones. One kind, T3, is active and is the functional form of thyroid hormone. The other kind, T4, is inactive and non-functional. The body has to convert T4 to T3 in order for it to be useful. The conversion of T4 to T3 happens in the liver, kidneys, brain, and even the gut through a process called deiodination

The most common cause of hypothyroidism is an autoimmune condition called Hashimoto’s thyroiditis

Symptoms of hypothyroidism include fatigue, hair loss, sleepiness, constipation, depression or low mood, swelling (puffy face, swelling in legs and arms), low heart rate, irregular menstrual cycles, infertility, and more. 

What is the keto diet?

The keto diet usually calls for eating very low carb (50 grams or less), high fat, and moderate protein. Foods that are allowed on a keto diet include:

  • Poultry & meats
  • Seafood
  • Dairy (as long as amounts don’t push you over the carb limit)
  • Nuts & seeds
  • Leafy greens
  • Low-carb veggies & fruits, like berries, green beans, etc. 

Let’s get into why I think keto and hypothyroidism are not a good mix. 

1 – Carbs are needed to make thyroid hormones work

To understand why keto and hypothyroidism don’t go well together, it’s important to understand how the thyroid works.

As I explained earlier, most of the thyroid hormone (T4) the thyroid gland makes is not ready to be used by the body. It has to be activated by the liver or other organs through a process called deiodination. The active form of thyroid hormone is T3. 

Research shows that:

  • Carbs are important for deiodination and they directly impact the levels of T3 in your body (1). 
  • And carbs in your diet directly impact how much of T4 gets converted to T3, the functional form of thyroid hormone (2). 

2 – Keto puts you in fight or flight, which is bad news for your thyroid 

When we don’t consume carbs, the body has to figure out how to make carbs to sustain blood sugar. The body relies heavily on stress hormones, cortisol, and adrenaline, to do this. 

Cortisol is responsible for increasing gluconeogenesis in the liver, a process that makes carbs from protein and fat. Adrenaline also supports gluconeogenesis. 

We know carbs directly help lower stress hormones and stress levels (3). So when you’re skipping carbs – as on a keto diet –  your cortisol levels can go even higher (4). 

Research also shows people with hypothyroidism tend to have higher levels of stress hormones, to begin with. We don’t want to make dietary changes that make those hormones even higher as can happen with the combination of keto and hypothyroidism (5)! Let’s explore why, next.  

So what happens when you have high stress hormones? 

Cortisol, a stress hormone, raises Thyroid Stimulating Hormone (TSH). 

A high TSH value corresponds with a sluggish thyroid, or hypothyroidism (6, 7). Cortisol also slows the conversion of T4 to T3 by directly impacting deiodinase activity and increases reverse T3 – an inactive form of T3 that can no longer be used by the body (8,9). 

A keto diet could put people with hypothyroidism at an even greater risk for high cortisol levels, which can then increase their risk for diabetes and heart disease. 

Research also shows that during stress, the body floods the blood with magnesium, This is a protective mechanism to help the body deal with stress. However, it also means that when you’re stressed, you’re losing a lot of magnesium in your urine (10).

Unfortunately, many of us aren’t getting enough magnesium to begin with. If you have hypothyroidism and spike your cortisol consistently,  you could be putting yourself at an even  greater risk for magnesium deficiency, especially if you are not getting enough through your diet. 

3 – Micronutrient deficiencies 

Keto requires keeping a carb intake of 50 grams per day or less. 

That means eliminating or only having tiny portions of nutrient-dense, carbohydrate foods. Carb-rich foods, like grains, lentils, fruits, and vegetables, are rich sources of vitamins and minerals. When eliminating carb-rich foods, we can put ourselves at risk for vitamin and mineral or micronutrient deficiencies. Two specific micronutrients I want to focus on are potassium and vitamin C. 


The most important mineral that we cannot get enough of through animal-based foods or fat-rich foods alone is potassium. 

The body needs potassium for pretty much everything it does, including supporting the kidneys, heart, muscles, and nervous system. 

People with hypothyroidism tend to also have low potassium levels, which means it’s even more important for them to include a variety of potassium-rich foods, like fruits and vegetables (11). 

The Institute of Medicine recommends 4,700 mg of potassium per day. The average American consumes 2,640 mg per day. That’s only 56% of the daily amount needed (12). 

The best way to get enough potassium is to eat plenty of fruits and vegetables, especially root vegetables like potatoes, sweet potatoes, beets, and squashes, or fruits, like bananas, apples, mangoes, etc. 

Eating plenty of fruits and vegetables is going to be hard to do if you’re on a low-carb diet. 50 grams of carbs is pretty limiting! 50 grams of carbs is about 2.75 cups of blueberries or 1.25 cups of rice or 3.25 cups of beets. 

Vitamin C 

Vitamin C is a water-soluble vitamin that we have to get through our diet because the body cannot make its own vitamin C. 

It is an antioxidant that reduces inflammation in the body and also helps regenerate other antioxidants in the body like vitamin E. Vitamin C is also involved in protein metabolism; and helps make connective tissue and some neurotransmitters. 

Vitamin C can also help with better absorption of thyroid medication and improve thyroid function by reducing TSH and increasing blood levels of T3 and T4 (13). 

Some experts assert that we need more vitamin C than is currently recommended, and I agree. Especially if you have hypothyroidism, I’d recommend that you aim for 200-400 mg of vitamin C a day – three times higher than the recommended daily amount (14). Making sure you’re getting enough vitamin C on a keto diet requires being very strategic. 

Can’t I just use a supplement? 

It seems simple, right? Just take a multivitamin and call it a day?

As a registered dietitian nutritionist (RDN), I recommend that my clients take a “food first” approach. That means aiming to get as many of your essential nutrients from where they occur naturally, instead of a capsule or pill. 

Not only do the nutrients in food tend to work together in harmony, but they’re also usually in the form that is most absorbable to the body – that’s not always the case with supplements. 

Supplements may be in a less absorbable form because that form is less expensive to produce. 

On top of that: there is no guarantee that a multivitamin will have everything you need. Most don’t. For example, most don’t have enough potassium or vitamin C. 

And if you try to cobble together enough of each vitamin and mineral from supplements alone, you may get too much! For example, too much potassium can cause your heart to beat irregularly and is extremely dangerous.  For all of these reasons,  I do not recommend trying to meet your potassium needs through supplementation. 

Having said that, there are times when a supplement is warranted. There is no one-size-fits-all approach to hypothyroidism; every person’s needs, preferences, and lifestyle are unique. That is why working with a knowledgeable registered dietitian can be so helpful in finding the eating plan that best suits your health conditions and goals. 

4 – Last but not least, it’s unsustainable 

The keto diet is unsustainable – when compared to a Mediterranean diet, the keto diet was less sustainable (15). The body is not designed to withstand an unbalanced diet that requires excessive willpower. 

Furthermore, the weight loss from the keto diet is likely going to come back. In fact, most people who attempt weight loss gain back the weight they lose and sometimes even more (16). 

Why go without carbs, when you can manage your health and hypothyroidism while eating carbs? 

Consider working with a registered dietitian who can help you find peace with food and improve your health, in a balanced and enjoyable way – including carbs! If you’re ready to chat, book your free consultation right here


While the keto diet can be an attractive tool for quick weight loss, it is not a sustainable or healthy method, especially if you have hypothyroidism. Trying a keto diet with hypothyroidism can put you at risk for worsening hypothyroid symptoms because carbs are directly involved in activating thyroid hormone. Keto can increase cortisol levels, which reduces thyroid hormone activation and increases TSH and it can be hard to get nutrients important for the thyroid gland on a keto diet. Consider working with a registered dietitian to help you get started! If you’re ready to chat, book your free consultation right here.  


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