Energy balance

Low-T3 syndrome in exercising women is induced abruptly when energy availability falls below a threshold of 19.0–25.0 kcal/kg lean body mass per day, regardless of exercise volume or intensity.

If you exercise regularly, your thyroid health depends on 'energy availability'—the calories you eat minus the calories you burn in exercise. If this number drops below roughly 30 kcal per kg of lean body mass, your body may switch into 'low-T3 syndrome,' slowing metabolism. To prevent this, you must either eat more to cover your exercise costs or reduce your exercise volume, as simply eating more without covering the exercise cost may not be enough if the gap is too wide.