Acquired autoimmune hypothyroidism is rare in early childhood, however, it must be considered in a 4 year old child with medical his-tory of delayed growth, increased somnolence, difficulty concentrating, and reduced activity. We report on the case of full clinical picture of severe hypothyroidism in one of the twins. Thyroid function deteriorated in one of the sisters, resulting in mental, motor and growth slowdown, remaining undiagnosed for about 2 years, while the other sister developed normally. In the reported case, severe hypothy-roidism and growth deficiency were accompanied by celiac disease. Initiation of L-thyroxine therapy resulted in an immediate response that increased the growth velocity by more than 2.2 times. This confirms the dominant role of thyroid hormones over celiac disease in the growth process, as the catch up effect started before gluten free diet was introduced.