An eight-year-old black male with severe growth and mental retardation presented with massive myxedema, hyponatremia, and hypothermia. History of hypothyroidism was confirmed by laboratory investigation. The hypothyroidism appeared to be secondary to organic pituitary insufficiency due to diffuse brain damage. Appropriate therapeutic response was achieved initially only with intravenous thyroxine therapy. This case is reported to illustrate the unusual association of secondary hypothyroidism with severe myxedema and hyponatremia in the pediatric age group. The physiopathology of hyponatremia in hypothyroid myxedema is discussed also.