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Thyroid Flashcards

1
Q

Describe the thryoid gland and the muscles around it

A
  • It secretes hormones directly into the Cell Interstitium and hence, into the blood stream.
  • This means in order to transport the large quantity of hormones secreted, it must have a large arterial and venous blood supply
  • It is found directly beneath the Cricoid Cartilage of the Larynx
  • It is comprised of two conical-shaped lateral lobes that lie on either side of the Larynx (Anterolateral Surface) and the Upper Tracheal Cartilage rings
  • The Lateral Aspects of the Thyroid Gland overlie the Anteromedial Aspect of the Carotid Sheath, notably the Common Carotid Artery
  • The 2 Lateral Lobes are joined anterior to the Trachea by an Isthmus of Thyroid Tissue (which lies directly on the Trachea), making the Thyroid Gland convex anteriorly
  • This Isthmus is found anterior to the 2nd, 3rd and 4th Tracheal Rings
  • Often, a tongue/extension of Glandular Thyroid Tissue arises from the Superior Border of the Isthmus and Extends superiorly towards the Hyoid Bone to form the Pyramidal Lobe of the Thyroid
  • This Pyramidal Lobe can sometimes be attached to the Inferior Surface of the Hyoid by the Levator Glandulae Thyroideae (which can sometimes be a strip of fibrous tissue instead of a muscle)
  • The Levator Glandulae Thyroideae/Fibrous Strip and Pyramidal Lobes, when present, are drawn out along the Embryological path of migration of the Thyroid, being remnants of the Thyroglossal Duct
  • The Inferomedial Surface of the Thyroid is moulded against the Trachea and Oesophagus, with the Recurrent Laryngeal Nerves travelling between the Trachea anteriorly, the Oesophagus Posteriorly and the Lobes of the Thyroid laterally
  • The Posteromedial Surface of the Thyroid is placed against the Cricoid and Thyroid Cartilage, in contact with the External Laryngeal Nerve (Motor Branch of Superior Laryngeal Nerve)
  • On its Anterolateral Surface, it is closely related to the Trachea and Cricoid Cartilages
  • On its Anteromedial Surface it is closely related to the Oesophagus
  • On its Lateral Surface, it is closely related to the Carotid Sheath
  • On its anterior surface, it is closely related with the Sternohyoid and Sternothyroid Muscles and the Pretracheal/Middle Layer of the Deep Cervical Fascia
  • This means that it moves up and down with the Larynx but cannot ascend above it as it is attached to the Larynx as these structures tether the Thyroid to the Larynx
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2
Q

What is pretrachial fascia

A
  • The Anterior Surface of the Lobes and Isthmus of the Thyroid are enclosed in the Pretracheal Fascia
  • This Fascia is attached to the Thyroid Cartilage and Cricoid Cartilage of the Larynx
  • Laterally, it is continuous with the Fascia of the Carotid Sheaths
  • Due to the Attachment of the Pretracheal Fascia to the Thyroid and Cricoid Cartilages, the Thyroid Gland moves up and down with the Larynx during Swallowing
  • Inferiorly, the Pretracheal is continuous with the Fascia over the Aortic Arch
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3
Q

What is pretrachial fascia

A
  • The Anterior Surface of the Lobes and Isthmus of the Thyroid are enclosed in the Pretracheal Fascia
  • This Fascia is attached to the Thyroid Cartilage and Cricoid Cartilage of the Larynx
  • Laterally, it is continuous with the Fascia of the Carotid Sheaths
  • Due to the Attachment of the Pretracheal Fascia to the Thyroid and Cricoid Cartilages, the Thyroid Gland moves up and down with the Larynx during Swallowing
  • Inferiorly, the Pretracheal is continuous with the Fascia over the Aortic Arch
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4
Q

Describe parathyroid glands

A
  • These are 4 Small Circular Glands (2 on each side), embedded into the Posterior Surfaces of the 2 Lobes, one superior and one inferior on each side
  • They are ductless endocrine glands that secrete PTH to maintain [Ca²⁺] (It is secreted when Hypocalcaemic to stimulate increased Ca²⁺ movement into the Blood)
  • The Superior Parathyroid Gland is relatively constant in position, lying in the middle of the Posterior Surface of the Thyroid Lobe bilaterally
  • The Inferior Parathyroid Gland can vary in position, lying near the Inferior Pole of the Thymus or even within structures found below the Lobe
  • The reason for this variable location is that the Inferior Parathyroid Glands develop alongside the Thymus in the neck
  • Therefore, as the Thymus Gland descends into the Anterior Mediastinum during development, it drags the Inferior Parathyroid Glands with it to end below the Superior Parathyroids despite forming superior to them
  • This means that the Inferior Parathyroid Gland can be found anywhere along the path of the Descend of the Thymus Gland
  • The Parathyroids share the same blood and nerve supply as the surrounding Thyroid Gland
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5
Q

Development of thyroid glands

A
  • The Thyroid Gland develops during Week 4 of Development
  • The Thyroid develops from the Floor of the Embryological Pharynx as an epithelial proliferation at the Foramen Caecum of the Developing Tongue -
    This leaves a depression in the Superoposterior region of the Tongue (Foramen Caecum) as an embryological remnant of this origin site
  • The Tongue originates from the 1st to 4th Pharyngeal Arches and Pouches
  • From this position in the Pharynx, cells of the Future Thyroid multiply and descend into the Neck, passing anterior to the Hyoid before looping up, under and posterior along the Inferior Surface of the Hyoid before descending along the anterior surface of the Thyrohyoid membrane to reach their adult position
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6
Q

Describe the development of the parathyroid gland

A
  • The Parathyroid Glands develop from the 3rd (Inferior Pair) and 4th (Superior Pair) Pharyngeal Pouch respectively but when they migrate towards and travel with the Thyroid as it descends, they flip positions to place the Superior Parathyroids above the Inferior Pair
  • By week 7, the Thryoid will have already migrated along the path of the Thryoglossal Duct to reach its final adult position in relation to the Tracheal Cartilages and Cricoid Cartilages
  • The Thyroglossal Duct is a duct that extends from the Foramen Caecum (In the Midline, between the Anterior 2/3rds and Posterior 1/3rd of the Tongue) and the Final Adult Position of the Thyroid
  • The Thyroglossal Duct eventually regresses and is obliterated at the end of Week 7 when the Thyroid is in its adult location
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7
Q

What is the arterial supply to the thyroid gland

A
  • The Arterial Supply to the Thyroid is mainly found on its Posterior Aspect whereas the Venous Drainage of the Thyroid is found on its Anterior Aspect
  • The Superior Thyroid Artery is the 1st branch of the ECA (Arises just superior to the Bifurcation of the Common Carotid Artery) and projects inferomedially, close to the External laryngeal Nerve to supply the Superior Aspect of the Thyroid
  • The Inferior Thyroid Artery arises from the Thyrocervical Trunk of the Subclavian Artery
  • the Superior Thyroid Artery supplies the Superior Parathyroid Gland and the Inferior Thyroid Artery supplies the Inferior Parathyroid Gland
  • This artery runs very deep, posterior to the Carotid Sheath, towards the Thyroid, where it branches to supply the Inferior Region of the Thyroid
  • The Inferior Thyroid Artery is much larger than the Superior Thyroid Artery and therefore, forms the main blood supply to the Thyroid
  • The Distal Branches of the Inferior Thyroid Artery are closely associated to the Recurrent Laryngeal and therefore, in Thyroid Surgery, one must take care to preserve the External Laryngeal Nerve when ligating the Superior Thyroid Artery and to preserve the Recurrent Laryngeal Nerve when ligating the Inferior Thyroid Artery (E.g During a Thyroidectomy)
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8
Q

Discuss some variants in arterial blood supply to the thyorid

A
  • In approximately 7-10% of the population, an additional artery is found anterior to the trachea, in the midsagittal plane and at the inferior aspect of the thyroid isthmus
  • This is known as the Thyroid ima artery and it supplies the thyroid
    gland and trachea
  • It is originally an embryonic artery, arising from the brachiocephalic trunk or the arch of the aorta
  • It is usually small or obliterated, providing minor blood supply to the Thyroid in some but can be substantial in the variant to the point where it is the main blood supply to the thyroid
  • When present, the Thyroid Ima Artery can overly the location of a Tracheostomy, after the Pharynx and Larynx become obstructed and hence, can be damaged during this procedure
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9
Q

Explain venous drainage of thyroid

A
  • The Thyroid is drained by a venous network on the Anterior surface of the Thyroid and eventually by 3 Pairs of Veins
  • The Superior Thyroid Vein ascends superolaterally to drain into the Internal Jugular Vein
  • The Middle Thyroid Vein projects laterally to drain into the Internal Jugular Vein
  • The Inferior Thyroid Vein(s) descend through the Superior Thoracic Aperture to drain into the Brachiocephalic Veins, posterior to the Manubrium
  • These 3 Veins also form extensive anastomoses on the Anterior Surface of the Thyroid
  • The Inferior Thyroid Veins can be variable in size, to the point where there might be a single Inferior Thyroid Vein lying either side to the midline of the Trachea instead of 2 Paired Veins
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10
Q

What are the innervations of the thyroid

A

The Thyroid is innervated by SNS Fibres from the Sympathetic Trunk and the PSNS Fibres from the Superior Laryngeal and Recurrent Laryngeal Nerves that arise from CN X

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11
Q

Describe lymphatic drainage of thyroid gland

A
  • Generally, these follow the course of Vascular Structures, draining predominantly into deep nodes around the Carotid Sheath and the Root of the Neck, with some draining into Mediastinal Nodes of the Thorax
  • The Thyroid mainly drains to Paratracheal and Pretracheal Nodes and then onto the Prelaryngeal and Tracheal Nodes (Thoracic Nodes)
  • These Deep Nodes are mainly found around the Jugulodigastric Node and hence, some Lymphatics from the Thyroid can drain to the Jugulodigastric Node via the Anterior Nodes of the Neck (Minor)
  • They may also pass directly into the Deep Cervical Nodes or directly into the Thoracic Duct or the Right Lymphatic Duct
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12
Q

Discuss the venous drainage of the thyroid

A
  • The Thyroid is drained by a venous network on the Anterior surface of the Thyroid and eventually by 3 Pairs of Veins
  • The Superior Thyroid Vein ascends superolaterally to drain into the Internal Jugular Vein
  • The Middle Thyroid Vein projects laterally to drain into the Internal Jugular Vein
  • The Inferior Thyroid Vein(s) descend through the Superior Thoracic Aperture to drain into the Brachiocephalic Veins, posterior to the Manubrium
  • These 3 Veins also form extensive anastomoses on the Anterior Surface of the Thyroid
  • The Inferior Thyroid Veins can be variable in size, to the point where there might be a single Inferior Thyroid Vein lying either side to the midline of the Trachea instead of 2 Paired Veins
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13
Q

Describe thyroid cysts

A
  • These are also known as Thyroglossal Cysts
  • This an occur anywhere along the developmental path of migration of the Thyroid Gland
  • These are persistent remnants of the Thyroglossal Duct from where the Thyroglossal Duct fails to close and become obliterated
  • These fill with fluid and can produce fluctuant, pain-free midline swellings
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14
Q

Describe lingual thyroid

A
  • Sometimes, small clusters of Glandular Thyroid tissue are found along the descent pathway of the Thyroid during development and may present as mid-sagittal swellings where some regions of the Thyroid do not migrate whilst others do
  • If Ectopic Glandular Thyroid tissue develops in the region of the Foramen Caecum on the Superoposterior Aspect of the Tongue, this region of Thyroid is called a Lingual Thyroid
  • This is also a midline, pain-free swelling like the Thyroid Cyst but is often found more superiorly and contains active glandular tissue vs just the remnant of a migratory duct
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15
Q

Describe Goitre

A
  • This is an Enlarged Thyroid Gland, mainly due to Dietary Iodine Deficiency, which produces a mid-sagittal swelling
  • The Thyroid Hypertrophy that occurs is usually benign but can turn Malignant
  • The Incidence of Goitre has been reduced significantly due to the introduction of Iodinized Salt
  • Other causes can be Congenital Hypothyroidism, Autoimmune Thyroid Diseases, Thyroid Cancers and Endocrine Disorders.- This reduces the functionality of the Thyroid and its ability to extract Iodine from the Blood, causing it to Hypertrophy in order to restore its activity
  • Here, the expanded Thyroid Gland can expand downwards through the Thoracic Inlet or Laterally (Following the Boundaries of the Pretracheal Fascia) but it is unable to rise any higher up in the neck than the Oblique line of the Thyroid Cartilage
  • This is because the attachment of the Thyrohyoid to this line and the Pretracheal Fascia tethering the Thyroid to the Larynx prevent the Gland from expanding anymore superiorly
  • Since the Thyroid is attached to the Trachea posteriorly, it also moves up and down the neck during swallowing with the Larynx and is usually pain free
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16
Q

Lymphadenopathy (non-thyroid lump)

A
  • Cervical Lymphadenopathy within the neck can be due to a range of infections of the Head and Neck or even Carcinomas (usually long standing health issues)
  • The Chain of Lymph Nodes in the Neck lie along the Internal Jugular Vein and can be palpated at the Anterior Border of the SCM
  • These swellings can be uni or bilateral and are usually, solid (not fluctuant as they are not fluid filled) and painful neck swellings
  • They are often parasagittal and not midline
  • You should always check for Symmetry in the cases of Cervical Lymphadenopathy as Bilateral Lymphadenopathy can indicate Systemic Infections/Inflammation or generally, Viral Infections (Except Mumps/Viral Parotitis) whereas Unilateral Lymphadenopathy can indicate localised conditions (E.g Cancers) or generally, Bacterial Infections
  • A Ring of Smaller Lymph Nodes lie around the Neck like a collar, running from the Occipital Region Posteriorly to the Submandibular and Submental Regions anteriorly, along the Inferior Border of the Mandible
17
Q

Describe brachial cysts and fistulae

A
  • These occur during development when the Rapidly Growing Second Pharyngeal Arch overlaps the Lower Pharyngeal Arches medially but the Ectoderm of the Pharyngeal Arch overlying its superficial surface fails to Fuse higher up and only fuses with the Ectoderm of the Lower Arches (Improper Fusion of the Branchial Arches or a Persistance/Lack of Obliteration of Branchial Clefts)
  • This leaves a Cervical Sinus (Potential Space with Ectodermal Walls, closed off Medially but open Laterally, causing the persistence of the Branchial Cleft) which can slowly fill with fluid and eventually present as a discrete swelling in the neck during Childhood
  • These Branchial Cysts protrude from the anterior border of the SCM (Lateral Neck), just inferior to the Angle of the Mandible for Lateral Branchial Cysts or Just Anterior to the Ear (Preotic Fistules - Needs to be differentiated from Preauricular Node Lymphadenopathy)
  • They often present as fluctuant, pain-free unilateral swellings (Pain-Free and Fluctuancy differentiates Lymphadenopathy from Branchial Cysts)
  • The Position of the Branchial Cyst along the Anterior Border of the SCM is dependent on which Branchial/Pharyngeal Arches fail to completely fuse

NSB (7 decks)

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