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Levator muscle of thyroid gland

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Last Updated: 23 November 2020

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Levator muscle of thyroid gland

Details
Insertionthyroid isthmus
Nerveansa cervicalis
Originthyrohyoid muscle
Identifiers
FMA13345
Latinmusculus levator glandulae thyroideae
TA22175
TA98A04.2.04.008

On seeing extreme variations in the gross anatomy of thyroid gland, Marshall stated that it was difficult to speak about normal thyroid gland. Thyroid gland in man is located in the lower half of front of neck. It consists of two lateral lobes which are connected by isthmus. In some cases, additional lobe is present, which is know as the Pyramidal Lobe. It forms a long pyramid which is attached by its base to the superior border of isthmus, usually at its junction with the left Lobe. Its apex is attached to the body of hyoid bone by a fibrous band, which sometimes contain muscular fibres which are know as Levator Glandulae Thyroideae. It is seldom midline position. Organogenesis of the thyroid gland in humans is often disturb, leading to a variety of morphological variations of the gland, such as hypoplasia, ectopy, hemiagenesis and agenesis. It is well known for its developmental anomalies which range from common to rare. Common anomalies include persistence of Pyramidal Lobe and thyroglossal duct cysts. Rare anomalies are agenesis or hemi-agenesis of thyroid gland, agenesis of isthmus alone or aberrant thyroid glands and origin of Levator Glandulae thyroidae from isthmus, of which very few cases have been reported in literature. According to Standring, musculus Levator Glandulae Thyroideae is a fibrous or fibromuscular band that stretches from the Pyramidal Lobe or upper border of isthmus of thyroid gland, usually on the left side, to the body of hyoid bone above. Base on observations of 210 levators, Mori classifies Levator Glandulae Thyroideae into five types; hyopyramidalis, b thyreopyramidalis, c thyreoglandularis, d Hyoglandularis, and e tracheoglandularis. In the present case, Levator Glandulae Thyroideae was see Pyramidal Lobe absent on the left side. It directly comes from the upper border of isthmus and go upto hyoid bone Hyoglandularis type according to Moris classification. Moreover, knowledge about fleshy slip of Levator Glandulae Thyroideae is also very important during neck surgeries, for preventing iatrogenic injuries.

* Please keep in mind that all text is machine-generated, we do not bear any responsibility, and you should always get advice from professionals before taking any actions.

* Please keep in mind that all text is machine-generated, we do not bear any responsibility, and you should always get advice from professionals before taking any actions

Embryology

The pharyngeal apparatus is responsible for formation of numerous parts of the head and neck region. During 3 gestational week, there is hypertrophy of endoderm in the midline of the primitive pharynx, arising from the first pharyngeal arch between tuberculum impar and copula; at point later referred to as foramen caecum. This thyroid primordium subsequently enlarges and is attached to the floor of the primitive pharynx by a hollow tube known as thyroglossal duct. The duct communicates with foramen caecum, which is caudal to tuberculum impar and rostral to copula. Thyroid primordium progresses to thyroid placode located at the base of the tongue; which subsequently forms thyroid diverticulum near the apical pole of aortic sac. Thyroid diverticulum then begins its descent towards its final pretracheal destination, passing through the anterior to laryngeal cartilages and hyoid bone. During this course, it maintains connection with the floor of the primitive pharynx via thyroglossal duct. Under normal circumstances, thyroglossal duct will degenerate and the diverticulum remains suspended in mesenchyme. Solid structure is invaded by vascular mesenchyme, which results in disruption of solid cellular arrangement. Subsequently, cells arrange themselves into a web of epithelial cords. Gland regain contact with aortic sac and subsequently bifurcate. This process is associated with rapid, thyroid stimulating hormone-independent, proliferation of thyroid progenitor cells. During 5 weeks of gestation, ultimobranchial body fuse with thyroid gland. Prior to this, ultimobranchial body, which is an endodermal derivative, was invaded by neural crest cells. These cells give rise to C-cells that participate in calcium homeostasis by producing calcitonin under hypocalcemic circumstances. The structure know as tubercle of Zuckerkandl is the only remaining structure at the point where two primitive structures merge. This tubercle can be seen in most adults in the posterior aspect of the gland. It is of clinical significance during thyroidectomies, as the nerve has a close relationship to recurrent laryngeal nerve. Around 7 gestational week, gland continues to enlarge and left and right lobes begin to form on either side of thyroid cartilage in proximal trachea. They are attached to each other by thyroid isthmus. In 10 gestational week, epithelial cords cluster together into smaller groups. Cells aggregate around lumen in single layer and form thyroid follicles. In 11 week, cellular differentiation occurs that results in an onset of thyroglobulin production and colloid can be seen in thyroid follicles. By 20 week, TSH levels begin to rise, resulting in production of thyroid hormones. In the final trimester, adult levels of thyroid hormone can be observed in the fetus and cellular proliferation responds to TSH.

* Please keep in mind that all text is machine-generated, we do not bear any responsibility, and you should always get advice from professionals before taking any actions.

* Please keep in mind that all text is machine-generated, we do not bear any responsibility, and you should always get advice from professionals before taking any actions

Neurovascular supply

Lymphatic plexus that arise from the thyroid gland also communicate with tracheal lymphatic plexus. They drain to Delphian lymph nodes that reside above thyroid isthmus. There is also subsequent drainage to paratracheal and pretracheal lymph nodes as well. There is also evidence supporting lymphatic drainage from thyroid gland to brachiocephalic lymph nodes near the thymus. Deep cervical lymph nodes receive lymph from the lateral part of the gland. This fluid is carried by lymph channels that travel along superior thyroid vein. There are also other lymph vessels arising from thyroids that bypass all lymph nodes and drain directly into the thoracic duct.

* Please keep in mind that all text is machine-generated, we do not bear any responsibility, and you should always get advice from professionals before taking any actions.

* Please keep in mind that all text is machine-generated, we do not bear any responsibility, and you should always get advice from professionals before taking any actions

Hormone synthesis

Table

HyperthyroidismHypothyroidism
AnxietyDepression
RestlessnessFatigue
TachycardiaBradycardia
Weight lossWeight gain
Thinning skinDry, itchy skin
Heat intoleranceCold intolerance
Frequent bowel movementsConstipation
Goitre possibleGoitre possible
Low TSHHigh TSH
High T3/T4Low T3/T4

There are six steps in synthesis of thyroid hormone, and you can remember them using the mnemonic ATE ICE: active transport of Iodide into follicular cell via Sodium-Iodide Symporter. This is actually secondary active transport, and Sodium gradient driving it is maintained by Sodium-Potassium ATPase. Thyroglobulin, large protein rich in Tyrosine, is formed in follicular ribosomes and placed into secretory vesicles. Exocytosis of Thyroglobulin into follicle lumen, where it is stored as colloid. Thyroglobulin is scaffold upon which thyroid hormone is synthesise. Iodination of Thyroglobulin. Iodide is made reactive by the enzyme thyroid peroxidase. Iodide binds to benzene ring on Tyrosine residues of Thyroglobulin, forming monoiodotyrosine then diiodotyrosine. Coupling of MIT and DIT give Triiodothyronine hormone and coupling of DIT and DIT give Tetraiodothyronine hormone, also know as Thyroxine. Endocytosis of iodinated Thyroglobulin back into follicular cell. Thyroglobulin undergoes proteolysis in lysosomes to cleave iodinate Tyrosine residues from larger protein. Free T3 or T4 is then release, and thyroglobulin scaffold is recycle. T3 and T4 are active thyroid hormones. They are fat-soluble and mostly carried by plasma proteins-Thyronine Binding Globulin and Albumin. While T3 is the more potent form, it also has a shorter half-life due to its lower affinity for Binding proteins. Less than 1 % of T3 and T4 is unbound free hormone. At peripheries, T4 is deiodinated to more active T3. T3 and T4 are deactivated by removing iodine. This happens in the liver and kidney. As T4 has a longer half-life, it is used in treatment of Hypothyroidism over T3 as its plasma concentrations are easier to manage.

* Please keep in mind that all text is machine-generated, we do not bear any responsibility, and you should always get advice from professionals before taking any actions.

Table2

HyperthyroidismHypothyroidism
AnxietyDepression
RestlessnessFatigue
TachycardiaBradycardia
Weight lossWeight gain
Thinning skinDry, itchy skin
Heat intoleranceCold intolerance
Frequent bowel movementsConstipation
Goitre possibleGoitre possible
Low TSHHigh TSH
High T3/T4Low T3/T4
* Please keep in mind that all text is machine-generated, we do not bear any responsibility, and you should always get advice from professionals before taking any actions

Function

At the cellular level, hormones promote an increase in the total number of mitochondria within cell, as well as the total surface area of mitochondria as well. This increase in quantity of mitochondria also corresponds directly to an increased metabolic rate observed in organism. Also, at the cellular level, thyroid hormones cause an increase in the rate of active transport of ions across cellular membranes. This is achieved by increasing activity of the sodium-potassium adenosine triphosphatase enzyme, which enhances the transportation rate of respective ions. This energy dependent activity is also supported by the aforementioned increase in the metabolic rate of organism. It is interesting to note that the thyroid gland is one of the first endocrine organs to begin functioning in utero. Aforementioned cellular changes along with other tissue specific functions of thyroid hormone have been shown to promote growth and development. It encourages absorption and metabolism of carbohydrates, as evidence by increased absorption of carbohydrates from gastrointestinal tract, optimization of glycolysis and gluconeogenesis, and increased activity of insulin. Furthermore, there is mobilization of lipids from their fat stores resulting in a decrease in the amount of triglycerides, cholesterol, and phospholipids have also been observed with increased thyroid hormone levels. The majority of cholesterol is excrete in feces. These events, as well as other tissue specific activities, result in an increase in the basal metabolic rate of organism. In the cardiovascular system, thyroid hormones have been shown to increase the rate and force of contraction of the heart, and, by extension, overall cardiac output. It has been proposed that this phenomenon is linked to increased oxygen demand associated with increased metabolic rate of organism. Increased blood flow is also necessary to dissipate excess heat generated from metabolic reactions that are occurring. Organism respiratory function also increase as there is a need to excrete excess carbon dioxide generated by heighten metabolic processes. Within the musculoskeletal system, thyroid hormones promote reaction of muscle fibres to neurotransmitters. Consequently, muscle fibres respond more vigorously with acceptable elevations in thyroid hormones. On the other hand, decrease in thyroid hormones would be associated with reduced muscular response to neurotransmitters. These hormones also have excitatory effects within the central nervous system; promoting either anxiety or depression with elevated or depressed hormone levels, respectively. The effects of hormones also extend to sexual reproduction; such that patients may experience loss of libido, impotence, or disorders of the menstrual cycle. Lastly, other endocrine glands are also affected by the activity of thyroid hormones. Thyroid hormones alter negative feedback mechanisms of other endocrine glands by increasing demand for other hormones at target tissue level. For example, by enhancing metabolism of glucocorticoid hormones in the liver results in lower circulating levels of hormones. This is detected by adrenal glands, which respond by increasing production of glucocorticoids.

* Please keep in mind that all text is machine-generated, we do not bear any responsibility, and you should always get advice from professionals before taking any actions.

* Please keep in mind that all text is machine-generated, we do not bear any responsibility, and you should always get advice from professionals before taking any actions

Sources

* Please keep in mind that all text is machine-generated, we do not bear any responsibility, and you should always get advice from professionals before taking any actions.

* Please keep in mind that all text is machine-generated, we do not bear any responsibility, and you should always get advice from professionals before taking any actions

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