Thyroid Embryology


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Thyroid Embryology and surgical anatomy:

Thyroid Embryology and surgical anatomy


EMBRYOLOGY Thyroid: 1 st endocrine gland to develop Day 24th of gestation Endodermal and ectodermal in origin  Proliferation of epithelial cells on the median surface of the floor of primitive pharynx  The site of this initial development lies at the junction of 1 st and 2 nd pharyngeal pouches between 2 key structures 1) Tuberculum impar 2) Copula This is known as FORAMEN CECUM.



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Cont.. Initially develops caudal to the “tuberculum impar” (median tongue bud) Foramen cecum begins ant to the copula, aka ‘ hypobranchial eminence ’ This endodermal outpouching descends caudal to form thyroglossal tract. Thyroid parenchyma develops by proliferation of the thyrocytes at the caudal end of the tract.

Parafollicular cells:

Parafollicular cells Aka C cells which are neuro ectodermal in origin.(neural crest cells). Secrete calcitonin (Ca regulation) From ultimobranchial body (last structure derived from the branchial pouches) Ultimobranchial body – from 5 th pharyngeal pouch

Parafollicular cells:

Parafollicular cells Migrating cells from the neural crest region infiltrate  ultimobranchial body (UBB) This structure (neural crest + UBB) = incorporated  thyroid gland UBB fuses with the thyroid gland and disseminates its cells into it These cells forms the postero lateral aspect of future thyroid gland.

Descent of thyroid gland:

Descent of thyroid gland Initial descent starts anterior to pharyngeal gut Thyroid is still connected to the tongue via the thyroglossal duct at this point. Later - tubular duct solidifies and subsequently obliterates entirely (during gestational weeks 7-10 ) Foramen cecum = opening of the thyroglossal duct into the tongue

Descent of thyroid gland:

Descent of thyroid gland Pyramidal lobe: Seen in up to 50% people This indicates persistence of the inferior end of the thyroglossal duct May be attached to the hyoid bone (~thryoglossal duct cyst) OR may be incorporated into a thyroglossal duct cyst

Descent of thyroid gland :

Descent of thyroid gland Further descent – anterior (ventral) to the hyoid bone and laryngeal cartilage During descent – thyroid forms its mature shape: median isthmus connecting 2 lateral lobes Descent COMPLETE at 7 th gestational week

Clinical correlations:

Clinical correlations Thyroglossal duct cyst duct fail to atrophy can form a sinus Rx – “Sistrunk” procedure (includes resection part of hyoid bone) 2) Ectopic thyroid can occur anywhere along the path of descent most common at the base of the tongue ‘ lingual thyroid ’

Clinical correlations:

Clinical correlations Accessory thyroid from remnants of thyroglossal duct may be functional but insufficient for normal function of thyroid

Inferior Parathyroid glands:

Inferior Parathyroid glands Arise from the dorsal wing of the third pharyngeal pouch. The third branchial pouch differentiates at gestational weeks 5-6, with the ventral wing becoming the thymus. The thymus and parathyroids both lose their connections to the pharynx at gestational week 7. The thymus then migrates caudally and medially, pulling the parathyroids with it;

Superior parathyroid glands:

Superior parathyroid glands Arise from the dorsal wing of the fourth pharyngeal pouch. Differentiating at gestational weeks 5-6. At gestational week 7, the glands lose connections with the pharynx and attach themselves to the thyroid gland, which is migrating caudally. Because of the lesser length of migration, the superior parathyroid glands are in a more constant location than the inferior parathyroids .


SURGICAL ANATOMY OF THYROID GLAND Cornerstone of safe and effective thyroid surgery is thorough training in and understanding of thyroid anatomy and pathology. Safe surgery requires a specific operative plan, progressing in a series of logical, orderly, anatomically based steps. The following are the important surgical land marks in thyroid surgery. 1.pyramidal lobe. 2.tubercle of Zuckerkandal . 3.ligament of berry. 4.external branch of superior laryngeal nerve. 5.inferior thyroid artery. 6.recurrent laryngeal nerve. 7.parathyroid glands. 8.pre tracheal fascia. 9.pre tracheal plane. 10. linea alba of neck. 11.thyrocarotid space.

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1.PYRAMIDAL LOBE: Distal end of thyroglossal tract persists as pyramidal lobe in 44%-61%. Located at the superior border of isthmus. Mostly located in left paramedian position. It is usually thick fibrous strand with little normal thyroid parenchyma. This is clinically important because at times this may be left behind in thyroid surgeries. 2.TUBERCLE OF ZUCKERKANDAL: Condensed thyroid tissue at the cricothyroid junction. Located posterolateral to thyroid gland. RLN enters larynx behind this. This is the location of maximum concentration of C cells which is important to be removed in cases like MCT. 3. LIGAMENT OF BERRY: This is condensation of pretracheal fascia. This firmly attaches thyroid posteriorly to cricoid cartillage and upper tracheal rings.

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Exposure of the thyroid gland is followed by careful dissection of the superior pole. Utilizing the avascular plane between the superior pole and the cricothyroid muscle identify and preserve the external branch of the superior laryngeal nerve. Medial retraction of the gland then allows dissection of the lateral aspect of the thyroid lobe. Protection of the recurrent laryngeal nerves and preservation of the blood supply to the parathyroid glands is best achieved by "capsular dissection," ligating the tertiary branches of the inferior thyroid artery on the gland surface.


THYROID IMAGING ULTRASONOGRAPHY: This is often the first imaging modality employed to evaluate a thyroid nodule. Advantages: 1.Readily accessible 2. Inexpensive 3. noninvasive 4.No radiation exposure. 5. Not time consuming.

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Ultrasonography is effective at delineating intrathyroidal architecture, distinguishing cystic from solid lesions. Determining if a nodule is solitary or part of a multinodular gland, and accurately locating and measuring a nodule. Has the added advantage of locating associated  lymphadenopathy  in the paratracheal region.

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Disadvantages: Unable to provide any functional information Cannot predict malignancy, and it is dependent on an ultrasonographer for the quality of images, regions of the neck covered, and interpretation. US scans can be misleading if lateral neck nodal regions are not fully inspected for nodal disease, and a sensitivity of only 37% .


CT & MRI CT is particularly useful in identifying and delineating the full extent of any cervical lymphadenopathy and the relationship of the thyroid to surrounding cervical viscera.  Lateral neck nodal disease sometimes not well investigated by US, as well as nodal disease in the parapharyngeal, retrolaryngeal, retrotracheal and retrosternal regions, are clearly and reliably seen on contrast CT.


Cont.. This radiographic combination of US and contrast CT produces an effective preoperative nodal map that enables us to direct our nodal dissection at the time of surgery. The added advantage with MRI is even better soft tissue assessment and detection of nodes as small as 4 mm where as CT can detect nodes of >1cm.

Radioiodine Uptake and Thyroid Scanning:

Radioiodine Uptake and Thyroid Scanning The thyroid gland selectively transports radioisotopes of iodine ( 123 I, 125 I, 131 I) and 99m Tc pertechnetate. Nuclear imaging of Graves' disease is characterized by an enlarged gland and increased tracer uptake that is distributed homogeneously. Toxic adenomas appear as focal areas of increased uptake, with suppressed tracer uptake in the remainder of the gland.

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Subacute thyroiditis is associated with very low uptake because of follicular cell damage and TSH suppression. Thyrotoxicosis factitia is also associated with low uptake. The functional features of thyroid nodules have some prognostic significance. 1.Cold nodules --- diminished tracer uptake and are usually benign. 2. These nodules are more likely to be malignant (5–10%) than so-called hot nodules.


Cont.. Hot nodules are almost never malignant. Thyroid scanning is also used in the follow-up of thyroid cancer.



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