Maybe you’re eating paleo, or ketogenic. You’re avoiding not just added sugar, but most or all grains, and you’re even limiting your fruit. You’re exercising, too.

But you’re not losing weight.

You can’t understand it. Your thyroid looks perfect on lab work (TSH and fT4 anyway). Maybe your sex hormones (like testosterone) are a little low, but nothing to account for all this. Your energy isn’t all that good either—not as good as it should be, for all the work you’re putting in.

You’re definitely not alone; while low carb diets work great for many people, for some, they either don’t work at all, or work only briefly. After that, the gains begin to plateau. If this is you, the problem most likely has to do with three hormones: free T3, reverse T3, and cortisol.

Euthyroid Sick Syndrome

Your thyroid is responsible for your body’s metabolism—which means, among other things, that it controls the uptake of glucose into the body’s cells.

Euthyroid Sick Syndrome means that thyroid hormone levels are all in lab reference range (that is, TSH, fT3, and fT4), but you’re functionally hypothyroid because your body is producing too much reverse T3. This is a physiologic adaptation when you’re sick: it’s your body’s way of putting on the breaks, telling you to rest so that you can recover from your illness. Usually this corresponds to an increase in cortisol, too: cortisol is the primary stress hormone, and it spikes in the presence of any kind of a stressor. Illness definitely qualifies.

What else qualifies? Hypoglycemia (low blood sugar).

Low Carb Diets and Blood Sugar Levels

There are three macronutrients: carbohydrates, fat, and protein. The primary macronutrient source of glucose is carbohydrates. The body cannot produce glucose from fat; it can only produce ketones, which the body can burn as energy (this is the whole idea behind the ketogenic diet). The liver and kidneys can produce glucose from protein (by gluconeogenesis), but they can only produce about half of the body’s daily glucose needs this way in the best of cases. The other half can be supplanted by ketones, an alternate energy source to glucose for many of the body’s tissues (including the brain). But in very low carb diets, this leaves little room for error.

If your body’s glucose requirements exceed the usual levels (because you’re sick, or perhaps stressed), and you’re on a low carb diet, your body has to cut corners somewhere. It does this by raising cortisol (encouraging gluconeogenesis) and also by converting T4 into rT3, instead of regular T3, thus slowing down your metabolism. This way, the overall demand for glucose decreases.

But if your metabolism slows down, you’re not losing weight. Plus, you’re likely pretty fatigued. Euthyroid Sick Syndrome means you have all the symptoms of hypothyroidism, even though the labs mostly look good, and prolonged high cortisol means you’re in the early stages of adrenal fatigue.

Reset Your Hormones

If this describes you, ask your doctor to check your free T3, your reverse T3, and your cortisol (this is best done with a saliva test, to give you the cortisol rhythm throughout the day, rather than with a blood test).

If these levels are off, the first thing to try is to add some extra whole food carbohydrates to your diet, such as whole grains or root veggies. Sometimes all your body needs is a little extra fuel!