Lingual thyroid is a rare congenital malformation that occurs more frequently in the female population. It occurs because of the error in transcriptional factors, the key for the normal differentiation of thyrocyte, so the thyroid gland tissue does not descend normally down the thyroglossal duct to the final position in the neck. Due to that, it can entirely or partially remain at the base of the tongue. This is the most frequent localization of the ectopic tissue while it can remain in the sublingual, suprahyoid and infrahyoid area as well. This disease can be diagnosed in the asymptomatic phase, as well as in the phase of compensatory and manifest hypothyroidism. In the ectopic thyroid gland, all diseases of the thyroid gland can occur as in the usual localization in the neck. The authors show a 6-year old patient, who had a routine medical examination for the inflamed throat, during which a vascular tumefaction was discovered at the base of the tongue. A cyst at the base of the tongue was suspected, but additional examination showed that it was an ectopic thyroid tissue marked as a lingual thyroid gland. Diagnosis of this disease starts with the laboratory analysis of the thyroid status. The next step involves scintigraphy of the thyroid gland with technetium-pertechnetate (99mTc) or radioactive iodine (123I). The therapy of the compensatory hypothyroidism is suppressive therapy with levothyroxine and in the manifest hypothyroidism it is hormone substitution therapy with levothyroxine. Although there are recommended age-related daily doses, they should not be accepted as final, but rather prescribed according to the individual thyroid status. .