We report the case of a 23-month-old girl who presented with poor growth and delayed attainment of gross-motor milestones. Elevated creatine phosphokinase (CPK) indicated rhabdomyolysis, ultimately attributed to severe, acquired autoimmune hypothyroidism. Growth data and bone-age suggest the onset of hypothyroidism occurred at or before 12 months of age. Acquired hypothyroidism is rare before age 3 years, and rhabdomyolysis due to hypothyroidism has not previously been reported as a cause of delayed gross-motor development in toddlerhood. Despite the early onset of hypothyroidism, cognitive function appeared to be unaffected. Adequate thyroid hormone replacement quickly normalized the CPK in our patient, and gross motor development rapidly improved. Although rare, rhabdomyolysis secondary to hypothyroidism should be in the differential diagnosis of delayed gross-motor development in infancy and toddlerhood.