Atypical thyroid tests in an athlete treated for hypothyroidism as the first symptom of pituitary dysfunction due to relative energy deficiency.

Relative energy deficiency in sport occurs in athletes who have limited energy availability. Its typical features include reversible suppression of the hypothalamic-pituitary-gonadal axis. In addition, it may be accompanied by hepatic resistance to growth hormone, leading to a decrease in insulin-like growth factor 1 and dysregulation of the hypothalamic-pituitary-thyroid axis. We present the clinical case of a 33-year-old athlete previously treated effectively for hypothyroidism, who presented with low thyroid-stimulating hormone, low free triiodothyronine, and normal free thyroxine. Based on diet and training interviews and further laboratory tests, dysregulation of the hypothalamic-pituitary-thyroid axis and reversible hypogonadism due to insufficiency of energy available to support energy expenditure were revealed. We also discuss here challenging diagnostic dilemmas that may appear in athletes of normal body weight but result from insufficient energy supply in relation to demand, and review the literature for the clinical course and possible mechanisms underlying the relative energy deficiency. Atypical thyroid function tests in athletes may be the first manifestation of reversible pituitary dysfunction due to relative energy deficiency. Typical symptoms of relative energy deficiency include reversible suppression of the hypothalamic-pituitary-thyroidal axis and hypothalamic-pituitary-gonadal axis. Relative energy deficiency may occur in all people who train intensively, regardless of their body weight and BMI, as well as in people who have rapidly lost weight as a result of a low-calorie diet and intense exercise.