The studies that we cited earlier focused primarily on children, and even though I had mentioned that thyroid problems do get worse with age, it is important to consider some of the research done on adult populations.
In fact, the earliest studies on the ketogenic diet and thyroid function were on adults. They evaluated how very low carb diets, that were not low in calories, affected thyroid hormone levels in adults.
Understanding The Findings
The findings were such that the lower the diet was in carbs, the more T3 lowered and reverse T3 elevated. This was also the same change noted in cases of starvation, as the body attempted to slow its metabolic rate (in order to store fat and stave off illness and death).
This study found that it was not just starvation, but that carbohydrate restriction itself was responsible for the changes in thyroid function5.
Another study performed, on completely healthy volunteers, showed the exact same findings. On a ketogenic diet, the thyroid function slowed. T3 levels dropped, and reverse T3 elevated. This change occurred even when the calories were not reduced6.