Background: Cases diagnosed with congenital hypothyroid- ism are considered to be at risk of thyroid nodule formation and subsequent thyroid carcinoma due to TSH overstimulation. Few cases has been reported hitherto. We are presenting two siblings diagnosed with congenital hypothyroidism due to dyshormogen- esis who developed thyroid papillary carcinoma during follow up.

Case Presentation: Case 1 was born at term as the first child of consanguineous parents. She was diagnosed with congenital hypo- thyroidism and on treatment with L-T4. She was admitted to our clinic at the age of 20 months. Tyhroid ultrasonography showed no pathogenesis. Perchlorate test was interpreted as organification defect. Genetic analysis revealed homozygous c.1477 G>A muta- tion in the TPO gene. L-T4 dose was adjusted acording to blood tests performed at regular intervals. Ultrasonographic follow-up was annually. At 17 7/12 years of age thyroid USG revealed an ovoid, hipoechoic 4*2 mm solid nodule, with punctual echogeni- ties, on the right lobe and an ovoid, 7,5*5,3 mm, solid nodule with smooth margin on the left lobe. Pathological examination of the fine needle aspiration sample from the nodule on the left lobe showed atypia with unknown significance. Thyroid gland was totally resected. Histopatological examination of nodule on the left lobe revealed papillary microcarsinoma. Second sibling of the fam- ily was on L-T4 treatment due to diagnosis of congenital hypothy- roidism at 13th day of life. Thyroid funcitons were followed with regular blood tests and thyroid USG annually. Genetic testing was compatible with that of elder sibling. At 7 9/12 of age, thyroid USG revealed a solid, 6-7 mm nodule on right lobe. It turned out to be a degenerated, cyctic, hypoechoic nodule with diameters of 11*6 mm. Pathological examination of the fine needle aspiration sample from the nodule showed atypia with unknown significance. Bilateral total thyroidectomy was performed. Papillary thyroid carcinoma was the diagnosis on pathological examination of the sample (T1bN0M0). Genetical analysis showed no BRAF muta- tion. Patient received radioactive iodine treatment. She is on fol- low-up without any morbidity.

Result: We wanted to emphysize the risk of thyroid carcinoma development in cases with congenital hypothyroidism due to familial dyshormonogenesis. Thereof, we believe regular follow- up of nodule development with thyroid USG is necessary in con- genital hypothyroidism.