A frequent objection to low carb diets is the low carb thyroid myth: the claim that they slow down thyroid function. According to research, this is not usually the case.
A frequent objection to a low carb diet is the low carb thyroid myth, claiming that low carb and ketogenic diets slow down thyroid function. There are numerous blog posts, articles and forum threads with personal accounts of low carbers stating that after a few months on a low carb diet, they developed hypothyroid symptoms such as fatigue, being cold, dry skin and weight gain.
My Personal Experience
This has not been my personal experience. My annual blood work shows that my TSH levels are stable at around 2 uIU/mL – they have been at this level when I was on a high carb diet, and stayed the same over the five years I’ve been eating low carb. I don’t check my T3 and T4 levels, because I don’t show any hypothyroid symptoms – I’m never cold (I actually tend to run a little hotter than most), I have good energy – no fatigue or brain fog, my hair is thick, nails are strong, and my skin is not excessively dry for my age (I’m in my forties).
However, I fully realize that my personal experience is very anecdotal and not real evidence, so I did some reading on the subject of a low carb diet and thyroid problems.
Low Carb Diets and Thyroid: What do the Experts Say?
Jimmy Moore reports that “two of the top low-carb nutritional health researchers in the world — Dr. Stephen Phinney and Dr. Jeff Volek — say this phenomenon with low thyroid while on a low-carb diet is “a myth” and has not manifested itself in any of the research subjects in their numerous studies of people who are properly following a well-formulated low-carb diet with adequate calories over the past three decades.”
Amber O’Hearn explains that while T3 hormone does tend to be lower on a low carb diet, this is not necessarily a bad thing and cannot be considered hypothyroid unless T4 is also low, TSH is elevated, and there are hypothyroid symptoms.
Low Carb Diets Heal Hormones
A very low carb diet is a diet that heals hormones, not a diet that disrupts them. A notable example is PCOS – Polycystic Ovary Syndrome, a relatively common hormone abnormality of reproductive-aged women. It has been established in numerous studies that a very low carb diet improves PCOS, and in fact even mainstream physicians now recommend a low carb diet to their PCOS patients.
Possible Reasons for Hypothyroid on a Low Carb Diet
So why do we hear of all these horror stories about people whose thyroid function was lowered after a few months on a low carb diet?
1. Not enough calories. My best guess is that these people are under eating, either intentionally restricting calories, or doing so unintentionally since a very low carb diet often makes you lose your hunger. Severe caloric restriction and prolonged under eating do affect thyroid function.
2. Rapid weight loss. Another possible explanation, related to the above, is rapid weight loss. If you’re overweight and suffer from insulin resistance, a low carb diet is probably the best way for you to lose weight effectively. But while weight loss is a good thing, rapid weight loss is hard on the body and can signal starvation, triggering thyroid issues. So make sure you eat enough calories on your low carb diet – there are many online trackers that you can use, such as fitday.com.
3. Not consuming enough fat. When Dr. Stephen Phinney and Dr. Jeff Volek talk about a “well-formulated low carb diet,” they mean a diet that contains enough calories and enough fat. It’s not that you need to go crazy on fat consumption, but when you reduce carbs to 20 grams per day or less, your energy needs to come from fat – protein is a good building block for the body, but it’s not a good source of energy. In a well-formulated low carb diet, at least 60% of calories should come from fat. If you don’t consume enough fat on a low carb diet, your body could arguably sense starvation, and lower thyroid function in an effort to conserve resources.
4. Lowering carbs too fast. It is possible, in my opinion, that a sudden reduction in carbs could trigger unwanted side effects, including a lowered thyroid function. So if you want to be on the safe side, you might want to lower your carbs gradually. This is the opposite of the Atkins approach, which includes a very low carb induction period, followed by gradual increase in carbs. So according to this approach, if you started out like me, consuming 300 grams of carbohydrate per day, you would lower carbs gradually, over a period of several months.
5. Very low carb diet. There’s some evidence that a ketogenic diet, which is a very low carb diet, does affect thyroid hormones, lowering T3 values. Whether this is a problem or not is debatable, but it’s possible in my opinion that for some people, a very low carb diet (fewer than 50 grams of carbs per day) is not a good idea, and they should stick with a moderately low carb diet (50-100 grams of carbs per day).
To sum things up: anecdotal stories aside, experts tell us that evidence does not show that a well-formulated, moderately low carb diet lowers thyroid function. So the low carb thyroid myth is just that – a myth.