For decades, it's been known that starvation and very-low-calorie diets decrease the level of circulating thyroid hormones in the bloodstream (3, 4). Low-carb and ketogenic diets have also been found to reduce levels of T3 in the bloodstream. This makes sense, as these diets have often been shown to induce physiological changes similar to fasting (5, 6).And although a low T3 level alone doesn't indicate hypothyroidism, a recent study suggests that reduced thyroid activity may occur in some susceptible individuals who adopt a very strict keto diet. In this 2017 study of epileptic children, 20 out of 120 subjects (over 16%) developed hypothyroidism within 6 months of starting a ketogenic diet therapy and were treated with synthetic T4 thyroid hormone (7).

Importantly, the classical ketogenic diet for epilepsy is severely restricted in carbs (typically less than 15 grams per day) and also low in protein. It's unknown whether adding in 10-15 grams of carb and increasing protein intake would have changed the results. Indeed, decades-old research in healthy people suggests that the magnitude of carb restriction coupled with calorie restriction influences how much T3 levels decline.

For instance, a 1976 study of obese people showed that T3 levels dropped significantly during a carb-free 800-calorie diet but didn't change much on an 800-calorie diet providing at least 50 grams of carbs per day (8). Yet even when calories aren't restricted, very-low-carb diets often lead to lower T3 levels in the blood. However, studies suggest that physical performance and metabolic rate don't appear to be affected by this reduction in T3 (9, 10, 11).

In one study of overweight type 2 diabetic adults who consumed a diet restricted to less than 20 grams of carbs daily, TSH levels decreased from 1.6 to 1.4 mIU/L, on average. This suggests that the study participants required less thyroid hormone, although T3 wasn't measured. In addition, they experienced several of the beneficial effects typically seen on keto diets, including weight loss and significant improvements in blood glucose control and insulin sensitivity (12).

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