Hashimoto's thyroiditis is a well-known cause of growth retardation in adolescence. It is less frequently seen in children and rarely seen in infants. A 4-year-old girl was referred to our clinic for a second opinion before starting growth hormone (GH) treatment. Linear growth had markedly declined in the past 2 years, with height -3.4 standard deviations. GH deficiency was complete. She had dry, gray-sallow skin and bloated abdomen, but no goiter. The parents reported fatigue and constipation. Hormonal evaluation revealed TSH 629.5 mIU/ml, free T4 0.08 ng/dl, and prolactin 17.2 ng/ml. Bone age was 2 years. Antibodies to thyroglobulin and thyroid peroxidase were positive, suggesting Hashimoto's thyroiditis. Brain magnetic resonance imaging showed anterior pituitary hyperplasia. After 3 years of L-thyroxine therapy, she was symptomless, her height was -0.6 standard deviations, and the TSH level was normal. Brain magnetic resonance imaging showed regression of the pituitary hyperplasia.