Thyroid Ultrasound > Introduction > Embryology


In the fetus, at 34 weeks of gestation, the thyroid gland appears as an epithelial proliferation in the floor of the pharynx at the base of the tongue. The thyroid then descends in front of the pharyngeal gut as a bilobed diverticulum through the thyroglossal duct. 

Over the next few weeks, it migrates to the base of the neck. During migration, the thyroid remains connected to the tongue by a narrow canal, the thyroglossal duct. TRH, TSH and T4 are produced beginning 18-20 weeks and T3 at 30 weeks. The portion of the thyroid containing the parafollicular C cells (neural crest origin), those responsible for the production of calcitonin, are derived from the 4th or 5th pharyngeal pouch. 

If I131 therapy is administered in a patient with Graves' disease, the fetus will not become hypothyroid due to thyroid gland destruction if it is less than 10-12 weeks old.

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