Under conditions of energy deficit or severe stress, the conversion of thyroxine (T4) to reverse triiodothyronine (rT3) is increased, resulting in a reduction in metabolic rate.

Study: Thyroid Axis Adaptations to Moderate Short-term Energy Restriction in Healthy, Young Women.

When these women ate much less food for a few days, their bodies made more reverse T3, a hormone that slows down metabolism to save energy—exactly what the claim says happens during low energy or stress.

Study: THE THYROID FUNCTION IN YOUNG MEN DURING PROLONGED EXERCISE AND THE EFFECT OF ENERGY AND SLEEP DEPRIVATION

When people are very tired and not eating enough, their bodies slow down their metabolism by changing a thyroid hormone into a less active form called reverse T3 — this study shows that exact thing happening.

Study: Euthyroid Sick Syndrome Precipitated By Rapid Weight Loss Following Semaglutide Initiation: A Case Report

When the body loses weight quickly and doesn't get enough energy, it slows down metabolism by changing thyroid hormones — like putting the body in energy-saving mode. This study shows that happens in people losing weight on a weight-loss drug.

Study: Effects of total energy withdrawal (fasting) on thelevels of growth hormone, thyrotropin, cortisol, adrenaline, noradrenaline, T4, T3, and rT3 in healthy males.

When people fast or don’t eat enough, their body makes more reverse T3, which slows down their metabolism to save energy—this study shows exactly that happening in people who went without food for 10 days.