When healthy young adults eat a low-carbohydrate diet with mostly polyunsaturated fat for seven days, their fasting T3 hormone levels drop compared to when they eat a low-carbohydrate diet with higher protein content.

We analyzed one assertion on this question and found that when healthy young adults followed a low-carbohydrate diet high in polyunsaturated fat for seven days, their fasting T3 levels dropped compared to when they ate a low-carbohydrate diet with more protein [1]. T3 is a thyroid hormone that helps regulate metabolism, and fasting levels are often measured to see how the body responds to changes in diet. This single observation suggests that the type of fat and protein in a low-carb diet may influence T3 differently. We did not find any studies that contradicted this finding. However, the evidence we’ve reviewed so far is limited to one group of people, one short time frame, and one specific dietary comparison. We don’t know if this effect lasts beyond seven days, applies to older adults, people with health conditions, or those with different activity levels. The evidence we’ve reviewed leans toward the idea that replacing protein with polyunsaturated fat in a low-carb diet may lower T3, but we can’t say why or how meaningful this change is for health. More research is needed to understand if this shift matters over time or in real-world settings. For now, if you’re eating low-carb and notice changes in energy or temperature sensitivity, it may be worth considering how much protein versus fat you’re consuming — but don’t assume this change is harmful or beneficial without more context.