October 25, 2017

Let's Talk About Thyroid -- Low-Carb or Ketogenic Diets and Thyroid Function (Pt. 2/3)

I left off last time saying we would look at the effects of low carb/ketogenic diets on thyroid function. Thyroid function is a hotly debated topic in the low carb world. While most people typically experience fat loss, better energy levels, and improved overall vitality on a low carb diet, in some individuals, measurements of thyroid-related hormones suggest that a low carbohydrate intake might be having adverse effects on the thyroid gland. Is it possible that a way of eating that has such wonderful benefits for so much of the body could be harmful for the thyroid?

The effect of low carb diets on thyroid health is quite the controversial issue. Some people following a low carb or ketogenic way of eating find that their T3 decreases after a while. At first glance, we might take this to mean that low carb causes a slowdown in metabolism, or maybe it has other negative downstream effects. On the other hand, physicians and researchers who’ve spent decades improving the lives of their patients with low carb and ketogenic diets have not reported adverse effects on thyroid function. So what’s the deal?

Well, this is why lab numbers are a guide and a good starting point, but they shouldn’t be the sole arbiter of your health and wellbeing. If your T3 is a bit lower after you’ve been following a low carb or ketogenic diet for a while but you feel great, then it’s not a problem. As I’ve written about with regard to fasting blood glucose and hemoglobin A1c, various lab values need to be interpreted somewhat differently for people following low carb diets than for those with a high carb intake.

Looked at through the lens of low carb, a decrease in T3 doesn’t automatically mean thyroid function is being compromised. In fact, it may be that the improvements in metabolic efficiency some people experience while being a “fat burner” rather than a “sugar burner” result in a heightened sensitivity to T3, and therefore a decreased need for higher levels. (World renowned keto researcher Stephen Phinney has written about this.) That is, their body gets the same effects from a lower level of the hormone. (Kind of like insulin sensitivity versus insulin resistance: when you are insulin sensitive, you need less of the hormone to provoke its effects than someone who is resistant.) To be clear, this is speculation, but it makes sense in light of the physiological mechanisms at work.

My friend Amber O’Hearn from ketotic.org and Tucker Goodrich from Yelling Stop have some good food for thought on this issue and both are recommended reading. (Fortunately, like every other writer on the planet, they are both much more succinct than I am.)

·      The Effect of Ketogenic Diets on Thyroid Hormones - especially good explanations for why lower T3 is not automatically problematic

For low carbers who experience a decrease in T3 and do become symptomatic, it’s possible this is the result of unintentional caloric restriction. Low carb diets tend to induce satiety more easily than high carb diets—meaning, people feel satisfied and stay fuller longer on fewer total calories when they’re eating low carb than when they ate high carb. (Some people, anyway. And particularly if their protein intake has increased.) For some people, this increased satiety might mean that, without even trying, they eat a lot less than they did before they switched to low carb. If this happens over the long term, it could affect thyroid function—as would any diet that results in a long term caloric deficit. The solution here isn’t necessarily to add back some more carbs; it’s to ensure adequate caloric intake. (Some people might do better with a slightly higher carb intake, though. Keto is not the only way for human beings to be healthy and fit, y’know.) Again, any diet with a long term caloric deficit might have the same result. It’s not because of the low carbs, per se, but rather, because of low total calories for an extended period of time. (Oddly enough, fasting is not the same thing as long-term caloric restriction, so we would not expect a slowdown in thyroid function from properly implemented fasting. Dr. Jason Fung explained this very nicely a while back on an episode of the Balanced Bites podcast.)

I am speculating again, but I suspect this is the culprit behind the (mostly female) people who report that a low carb or ketogenic diet “destroyed their thyroid,” or led to adrenal fatigue (which is a questionable phrase anyway). As I described in my rant about fasting, it seems to me that the people who experience these outcomes on low carb or keto are those who are starting from a baseline of being high-stress individuals. On top of that, they do a great deal of exercise, don’t get enough sleep, and typically dramatically under-consume total calories. They don’t mean to do this; it’s just that decades of brainwashing have led them to think that 3 ounces of chicken breast and a pile of romaine lettuce is sufficient to “refuel” after an intense workout. So it’s not exactly a big shock when they feel better adding sweet potatoes, white rice, or even oatmeal to their diet. Maybe it’s the calories, or maybe it is the carbs. Some people really, truly, do feel better when they increase their carbs. I have nothing against that. (And if it angers or offends you in some way, well, that says a lot more about you than it does about them.) Truly, not everyone needs to be keto 24/7 to look, feel, and perform their best.

Of course, this being said, people could experiment with upping calories from protein and fat before they try increasing carbs, but it doesn’t threaten my worldview if they’d rather just go with the carbs.     

Next time: deep-dive into my ongoing personal hell adventure with hypothyroidism, and the many lessons I've learned and am still learning along the way.

Disclaimer: Amy Berger, MS, CNS, NTP, is not a physician and Tuit Nutrition, LLC, is not a medical practice. The information contained on this site is not intended to diagnose, treat, cure, or prevent any medical condition and is not to be used as a substitute for the care and guidance of a physician. Links in this post and all others may direct you to amazon.com, where I will receive a small amount of the purchase price of any items you buy through my affiliate links.


  1. Mild hypothyroidism - lower T3 and higher TSH - is associated with longevity in many populations; despite association with higher LDL, there's also lower CVD risk (as long as it's not extreme hypothyroidism)

    1. Hey George! Thanks for reading. I'm a fan of yours. :)
      I can see why lower thyroid hormones would be associated with longevity. It's sort of like having your power level set to "low," or just "power save mode." This seems great as long as someone is asymptomatic and feels well. For me, my TSH and T3 were in the basement and I felt awful. I think we need to focus less on the numbers and more on how people feel -- something I touched on in the previous post and will also address in the next one. But when you look at everything thyroid hormones do, it makes sense that being on the low side could have a beneficial effect on longevity.

  2. Replies
    1. Soon! Later this week or sometime next week. It’s already written but needs some editing.

  3. Devil's advocate - I suspect the "epidemic" of low thyroid results from the widespread adoption of low fat diets, along with most of the rest of the metabolic disasters. Yes I agree it is most common in women probably resulting from a "low everything" diet. What were the affected low carbers eating *before* they went low carb, and for how long? A fruit smoothie for breakfast and three lettuce leaves and a radish (while smiling blissfully to camera)for dinner?

    Devils advocate 2 - my thyroid remained resolutely normal for most of my life, until about ten years of low carb/paleo/keto when it suddenly went HIGH

    Devils advocate 3 - my father also went hypERthyroid when he was young, and ate a high everything diet most of his life. When he subsequently went hypOthyroid in old age this was almost certainly a result of having most of the gland hacked out earlier.

    BTW did you know you have an imitator? Well not exactly but I thought the title was a tad similar


    chris c

  4. Thanks for this article. Started keto about 6 weeks ago. Feeling awful for the past 4 weeks. Got my Free T3 measured this week, and it's at the bottom of the normal range. I suspect this may be why I'm feeling bad. Not sure whether to try to switch from synthroid to NDT and/or to add some carbs back into my life regularly. I just know I can't keep gaining wait, feeling anxiety, having depression, eye twitching, irregular bleeding... this has been CRAZY.