A rare disorder with characteristics of pseudohypertrophy of muscles due to longstanding hypothyroidism. Prevalence is unknown. The syndrome usually presents between 18 months and 10 years but has been reported at earlier ages including during the neonatal period. Patients present with clinical features of hypothyroidism, including decreased activity and increased sleep, feeding difficulty and constipation, prolonged jaundice. Pseudohypertrophy involves the muscles of the extremities, limb girdle, trunk, hands and feet but is more prominent in the limbs, resulting in an athletic appearance. Hypothyroidism, or thyroid hormone deficiency, may be congenital and may be permanent or transient.