Embryological development explains the positions, anatomical variations and congenital conditions of the thyroid and parathyroid glands.
Thyroglossal duct and descent
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The thyroglossal duct develops from the median bud of the pharynx.
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The foramen caecum, at the junction of the anterior two-thirds and posterior one-third of the tongue, is the vestigial remnant of the duct.
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The initially hollow structure migrates caudally, passing closely alongside, and sometimes through, the developing hyoid cartilage.
Parathyroid development and position
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The parathyroid glands develop from the third and fourth pharyngeal pouches.
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The inferior parathyroid gland and thymus arise from the third pouch. During descent, the thymus carries the associated parathyroid gland with it, explaining its usual position below the superior parathyroid.
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The inferior parathyroid may occur anywhere along this line of descent.
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The superior parathyroid gland and ultimobranchial body arise in the fourth pharyngeal pouch; the developing thyroid lobes amalgamate with these structures.
Parafollicular cells
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Parafollicular cells, also called C cells, originate from the neural crest and reach the thyroid through the ultimobranchial body.