Under-fueling, RED-S & thyroid function

Relative energy deficiency in sport (RED-S) causes the body to go into energy saving mode. This is why athletes with RED-S often have a stable body weight despite under-fueling (for more information on RED-S, click here). One of the ways the body saves energy is by impacting thyroid function. Read on to find out more.

The thyroid gland sits just below the Adam’s apple and synthesizes the thyroid hormones: T₄ and T₃. A hormone from the brain called “thyroid stimulating hormone” stimulates the thyroid gland to secrete these hormones. Thyroid hormones help to regulate various functions such as metabolism, body temperature and heart rate. When an individual has low levels of thyroid hormones this is termed “hypothyroidism” whereas high levels of thyroid hormones is termed “hyperthyroidism.” Both are medical conditions that require attention.

High and low levels of thyroid hormones can be caused by various things, and not all are related to nutrition. However, athletes that are under-fueling can display a variation of “euthyroid sick syndrome.” In this situation, hypothyroidism is due to inadequate calorie intake and high levels of exercise. For these athletes, that brain suppresses thyroid hormone synthesis. Typically, low levels of thyroid stimulating hormone, T₄ and T₃ will be seen in the blood. With low levels of thyroid hormone the body slows down its metabolic rate, so that less energy is required. This can lead to symptoms like feeling exhausted and sluggish even if sleeping more and feeling colder than everyone else around you.

For athletes with low thyroid levels due to RED-S, treating the under-fueling is necessary for improving thyroid function. If thyroid pills are given to these athletes to increase the levels of thyroid hormones, this could worsen RED-S as this would increase energy expenditure and create a larger energy deficit. As such, treatment should focus on increasing calorie intake and in some situations, also reducing the amount of exercise.

Under-fueling impacts various aspects of health. You should be more concerned about the negative aspects of under-fueling than over-fueling. To perform at your best, adequate fueling is key.