Editing Endocrine system (section)

===Thyroid gland===
The [[Thyroid|thyroid gland]] develops from two different clusterings of embryonic cells. One part is from the thickening of the pharyngeal floor, which serves as the precursor of the thyroxine (T<sub>4</sub>) producing [[Follicle (anatomy)|follicular]] cells. The other part is from the caudal extensions of the fourth pharyngobranchial pouches which results in the parafollicular calcitonin-secreting cells. These two structures are apparent by 16 to 17 days of gestation. Around the 24th day of gestation, the foramen [[cecum]], a thin, flask-like [[diverticulum]] of the median [[Primordium|anlage]] develops. At approximately 24 to 32 days of gestation the median anlage develops into a [[bilobed structure]]. By 50 days of gestation, the medial and lateral anlage have fused together. At 12 weeks of gestation, the fetal thyroid is capable of storing iodine for the production of [[Thyrotropin-releasing hormone|TRH]], [[Thyroid-stimulating hormone|TSH]], and free thyroid hormone. At 20 weeks, the fetus is able to implement feedback mechanisms for the production of thyroid hormones. During fetal development, T<sub>4</sub> is the major thyroid hormone being produced while triiodothyronine (T<sub>3</sub>) and its inactive derivative, reverse T<sub>3</sub>, are not detected until the third trimester.