Thyroid agenesis is the complete absence of the thyroid gland from birth.

Congenital hypothyroidism is a condition that results from inadequate thyroid hormone production from birth; the worldwide incidence is 1:3,000 to 1:4,000. The condition can be categorized in terms of etiology as thyroid agenesis (22–42%), ectopic thyroid gland (35–42%), or gland in situ abnormalities (24–36%).

Thyroid hemi-agenesis is defined as the lack of development of one thyroid lobe, or one thyroid lobe and the thyroid isthmus.

Hemi-agenesis of the thyroid gland is a rare condition — only 300 cases had been reported up to 2010, and it was first reported by Handsfield-Jones in 1866.

In the general population the prevalence estimated from screening ultrasound studies is approximately 0.06% (ranging 0.05% to 0.2%), with the left thyroid lobe being more common. It is more common in females, with a 3:1 female to male ratio. The higher frequency of hemi-agenesis occurs in the left lobe in 80% of the reported cases and is associated with isthmus agenesis in 50% of the cases.