Low carb is so hot right now. Some days it feels like very man, woman and their dog are low-carbing their way to leaner bodies and better health. Admittedly, I’ve even been on the low-carb bandwagon myself in recent years. And as a nutritionist, any way of eating that helps us to all reduce our overconsumption of refined, processed carbohydrates gets the tick of approval from me.

But is low carb right for everyone?

To be honest, there is no one diet that is right for everyone – we all have unique bodies with different needs. To add, the way of eating that works for you today might be different from the way you need in the future. Things change. The best barometer is to listen to your body and notice the messages it is sending you, for example, if you’ve been trying low-carb eating and noticed a change in your health and wellbeing for the worse, well, that’s something to pay attention to. The truth is, you’re not alone.

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The low carb diet & the thyroid

Contrary to a popular opinion, many people actually find that low-carb eating ends up making them feel worse than they did before they started on it.

The common story goes – you start low carb diet after hearing all the hype about how wonderful it is for weight loss/energy/mood/cognition/all of the above, only to find after a few weeks or months into the diet you feel more tired, cranky, lethargic, or irritable than you did before*. Maybe you even develop other symptoms you didn’t have prior to making the switch, such as feeling cold, hair loss, sleeping troubles, joint pain, loss of the menstrual period in women, or digestive troubles. Ultimately, you are left wondering why low carb isn’t working for you when your partner/best friend/colleague seems to be thriving on it?

(*Note: fatigue is normal in the first few days or first week of switching to low-carb eating. In fact, many call this phase and its associated symptoms the ‘keto-flu’ in reference to the cluster of symptoms that resemble the flu which occurs when switching from carb-burning to fat-burning for energy. Most people move past keto-flu symptoms after a few days or a week, but some people may never shift out of this state)

This response could, in part, be related to changes in thyroid function that can occur with low carbohydrate intake.

Both clinically and in the scientific research we see evidence of an interaction between dietary carbohydrate intake and thyroid hormone levels. Although it is not entirely clear how exactly low carbohydrate intake impacts thyroid hormone levels, there are a few clues and a couple of theories.

As is to be expected with anything related to hormones in the body, it’s complicated.

Low-carb & your thyroid (and your insulin levels)

The thyroid, master of metabolism, exerts its influence via the active thyroid hormone T3 (triiodothyronine). However, the thyroid gland produces inactive thyroid hormone, T4 (thyroxine) is significantly higher amounts. For the thyroid to pull any metabolic strings, T4 must be converted into T3 by specific enzymes.

Some research suggests that there is an interaction between insulin and the enzymes that convert T4 to T3 (1, 2, and these cell studies – 3, 4), showing that insulin may have some role to play in the conversion of inactive to active thyroid hormones.

When we limit carbohydrate intake, our insulin levels drastically decrease. This is why low-carb eating is so great for diabetes type II and obesity – it can reverse the metabolic dysregulation seen with the overconsumption of refined carbohydrates and sugary foods. Great news for those with insulin resistance!

But sadly, it’s not such great news for the thyroid. If insulin is involved in thyroid hormone conversion, as the evidence leads us to believe, then plummeting insulin levels may impact T3 levels. Over time, low T3 levels are associated with low thyroid function, low metabolism, fatigue, low mood, etc., etc., the list of sluggish thyroid problems goes on.

Low-carb & your thyroid (and your cortisol levels)

On the other hand, when our blood sugar levels are low, as they typically are in someone eating a low-carb diet, we may see reactively high cortisol levels. One of cortisol’s primary functions is to maintain adequate blood sugar levels in case of a stress-emergency (i.e. fight or flight). Therefore, cortisol levels may start to rise if blood sugar levels drop too low.

Cortisol talks to the liver and kidneys, instructing them to release glycogen (the storage form of glucose) into the blood so that blood sugar levels can rise to levels able to support us through a fight-or-flight response. When the kidneys and liver announce that there isn’t any stored glycogen because of low-carb intake, more and more cortisol is pumped out of the adrenal glands to try and rectify the situation.

Thyroid function is very sensitive to influence from the activity of the adrenal glands and cortisol (5). Typically, high cortisol levels correlate with low thyroid hormone levels (5, 6, 7), however, the relationship is highly nuanced and can be different depending on the type and duration of the stressor. But for the sake of simplicity, high cortisol usually = lower T3 levels.

But my thyroid tests are fine (according to my GP)?

Thyroid screening tests ordered through your doctor typically only measure TSH (thyroid stimulating hormone). If you’re very lucky, your test might also include T4 (thyroxine). But TSH and T4 only give us a small glimpse into what is really going on with thyroid function.

For an accurate interpretation, thyroid tests really need to include T3, and depending on the case, reverse T3 and thyroid antibodies as well. Without at least the first three markers (THS, T4, T3), it is nearly impossible to know what is going on with thyroid function.

In many cases, TSH and/or T4 can be absolutely ‘normal’ and within range, while T3 levels are looking suspiciously low.

A partial ‘hibernation’ effect

It is well documented that calorie restriction reduces T3 levels (8, 9, 10). The body tends to convert T4 into reverse T3 (inactive thyroid hormone) instead of T3, in what is believed to be an energy-preserving adaptation of the thyroid gland in response to perceived famine or starvation (11).

Essentially, the thyroid goes into hibernation as if it is facing a long, cold winter without food, a bit like a bear.

Some are quick to attribute the effects of low carb eating on thyroid hormone levels to an energy deficit – assuming, of course, that there is an actual energy deficit.

But interestingly, studies have shown that the low T3 effect occurs when people maintain sufficient energy intake but consume very low amounts of carbohydrates (9). This suggests that the change seen is not merely a result of low energy intake alone.

To add, T3 levels might only rise again in the presence of moderate to high levels of carbohydrate intake (but not with fat or protein) (9), highlighting the pivotal role dietary carbohydrates appear to play in the regulation of thyroid function. In this study, carbohydrate intake at 50g partially restored T3 levels, while 160g was needed to fully restore T3 levels.

For context, 160g of carbs is approximately equal to 1 cup of legumes (~40g), 2 cups starchy vegetables (~40g), 1 cup of rice (~40g), and 2 pieces of fruit (~40g) – in other words, quite a bit of carbs.

However, low carbohydrate intake may trigger a similar hibernation effect. Very low carb diets, like the ketogenic diet, essentially mimic the metabolic effects of fasting (12), which may confuse the body and thyroid into thinking there is a lack of food.

Maybe you went too hard or too fast?

Some suggest that low-carb relate thyroid changes can occur as a result from making the switch to low carb eating too quickly or from eating too few carbs overall. The sudden change that many people make – going from regular carb intake to low-carb intake virtually overnight can trigger the ‘hibernation effect’, as was discussed above. To add, there may be a ‘sweet spot’ for each of us regarding the overall amount of carbs we each need before the thyroid in impacted – as shown by the fact that some low-carbers thrive on this way of eating for many, many years.


To explore these possibilities more, try reintroducing carbs into your meals again until your symptoms reside, then gradually reduce your intake (we’re talking small changes over a couple of weeks) to find your own personal ‘sweet spot’ and prevent the overnight hibernation effect from kicking in.

Not all carbs are created equally

Although you might need more carbs to support your thyroid, don’t mistake this as a free-pass to eat chips, cakes, and cookies as you desire. The best choice is to aim for complex carbohydrates – which I like to refer to as ‘slow carbs’ as they take a long time to break down in the digestive system thanks to their high fibre content, providing a good source of carbohydrates to the body in a slow release form (ergo, no nasty blood sugar spikes). The other bonus is that slow carbs are very nutrient dense, providing not only carbs but also a whole slew of other essential nutrients needed for thyroid health, such as selenium and zinc.

Aim to include ½ cup of one of the following with each meal:

  • Starchy vegetables – sweet potato, parsnip, pumpkin, potato
  • Quinoa
  • Legumes – lentils, beans, chickpeas
  • Rice – white is ok alongside other foods like vegetables and meats, otherwise go for brown
  • Buckwheat
  • High fibre, fresh fruit – skin on is best!

Photo by Anna Pelzer on Unsplash


In short, the amount of carbohydrate consumed in the diet appears to have an impact on the level of our active thyroid hormone, T3 (triiodothyronine).

This impact may only apply to a specific subgroup of the population, as clinically and anecdotally, many people report excellent long-term results on a low-carb diet.

If you know you have an existing hypothyroid condition or suspect that your low-carb eating may be having less than desirable results, it is advisable to maintain moderate carbohydrate intake. The amount needed may be different for each person – some may need levels as high as 160g, while others might be able to maintain a lower intake at 50g, or for you, it might be somewhere in between.

Or, if you’re eating low-carb and loving it, with minimal unwanted side effects, then good for you! Enjoy you low-carb life and continue on as you were.

And if you ever need a helping hand to find your carb-sweet spot or for other matters relating to the thyroid, feel free to get in touch or book an appointment for more nutritional and naturopathic support from yours truly!