European Journal of Molecular & Clinical Medicine

During the third week of gestation, the thyroid gland begins to form on the floor of the rudimentary pharynx. The thyroglossal duct carries the gland from its origin at the foramen caecum at the base of the tongue to its eventual resting place in the neck. These cysts of the thyroglossal duct and the unusual ectopic position of thyroid tissue at the base of the tongue (lingual thyroid) are both explained by this anatomical trait. Synthesis of thyroid hormone typically starts during the 11th week of pregnancy.
Thyroid follicular cells form the follicles seen in a fully developed thyroid gland, and the colloid they release is a proteinaceous fluid rich in thyroglobulin.⁹