The thyroid gland is one endocrine organ uncovered in the neck, it is responsible because that regulating the body metabolic price via hormone it produces. In this article, we will be looking in ~ its anatomy, its to move structure, that is endocrine physiology and also its clinical relevance.

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Anatomy

The thyroid gland is a ductless alveolar gland discovered in the anterior neck, just below the laryngeal prestige (Adam’s apple). The is about butterfly-shaped, v two lobes wrapping approximately the trachea and also connected in the center by one isthmus. The thyroid gland is not usually palpable.

It is offered by superior and inferior thyroid arteries, drained via superior, middle and inferior thyroid veins and has a affluent lymphatic system.

You can read more about the anatomy the the Thyroid gland here.


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Fig 1 – Anterior check out of the neck, reflecting the anatomical place of the thyroid gland


Cellular Structure

The duty of the Thyroid gland is to produce and store thyroid hormones. Thyroid epithelia type follicles filled through colloid – a protein-rich reservoir the the products needed because that thyroid hormone production. This follicles variety in dimension from 0.02-0.3mm and also the epithelium might be basic cuboidal or basic columnar.

In the spaces between the follicles, parafollicular cells deserve to be found. These cells secrete calcitonin, i beg your pardon is affiliated in the regulation of calcium line in the body.


Function

The thyroid gland is one of the main regulators of metabolism. T3 and also T4 commonly act via atom receptors in target tissues and initiate a variety of metabolic pathways. High level of lock typically cause these processes to occur quicker or an ext frequently. Metabolic processes boosted by thyroid hormones include:

Basal Metabolic RateGluconeogenesisGlycogenolysisProtein synthesisLipogenesisThermogenesis

This is achieved in a variety of ways, together as increasing the dimension and number of mitochondria in ~ cells, enhancing Na-K pump activity and enhancing the presence of β-adrenergic receptor in tissues such as cardiac muscle.


Thyroid Hormone Synthesis

There space six steps in the synthesis of thyroid hormone, and also you have the right to remember them using the mnemonic ATE ICE:

Active transport that Iodide into the follicular cell via the Sodium-Iodide Symporter (NIS). This is actually second active transport, and also the sodium gradient driving that is preserved by a Sodium-Potassium ATPase.Thyroglobulin (Tg), a big protein affluent in Tyrosine, is formed in follicular ribosomes and also placed into secretory vesicles.Exocytosis the Thyroglobulin right into the follicle lumen, where it is stored as colloid. Thyroglobulin is the frame upon i m sorry thyroid hormone is synthesised.Iodination of the Thyroglobulin. Iodide is make reactive by the enzyme thyroid peroxidase. Iodide binds to the benzene ring top top Tyrosine residues of Thyroglobulin, creating monoiodotyrosine (MIT) climate diiodotyrosine (DIT).Coupling of MIT and also DIT provides the Triiodothyronine (T3) hormone and coupling the DIT and DIT offers the Tetraiodothyronine (T4) hormone, additionally known as Thyroxine.Endocytosis the iodinated thyroglobulin earlier into the follicular cell. Thyroglobulin experience proteolysis in lysosomes come cleave the iodinated tyrosine residues from the larger protein. Totally free T3 or T4 is then released, and also the Thyroglobulin scaffold is recycled.

T3 and T4 room the energetic thyroid hormones. They room fat soluble and also mostly lugged by plasma proteins – Thyronine Binding Globulin and Albumin. When T3 is the an ext potent form, it additionally has a shorter half-life because of its reduced affinity because that the binding proteins. Much less than 1% of T3 and also T4 is unbound cost-free hormone. At the peripheries, T4 is deiodinated come the an ext active T3.

T3 and also T4 room deactivated by removing iodine. This happens in the liver and also kidney. Together T4 has actually a much longer half-life, that is used in the treatment of hypothyroidism end T3 as its plasma concentrations are easier to manage.


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Fig 2 – overview of the synthetic of thyroid hormones


Thyroid Hormone Release

Thyroid hormones are released as component of the hypothalamic-pituitary-thyroid axis. The Hypothalamus detects a low plasma concentration the thyroid hormone and releases Thyrotropin-Releasing Hormone (TRH) into the hypophyseal portal system.

TRH binding to receptors uncovered on thyrotrophic cell of the anterior pituitary gland, leading to them to release Thyroid Stimulating Hormone (TSH) right into the systemic circulation. TSH binding to TSH receptor on the basolateral membrane the thyroid follicular cells and also induces the synthesis and also release the thyroid hormone.


Clinical relationship – Goitre

A Goitre is the clinical term because that an enlarged thyroid gland. The body organ swells approximately a palpable, and also often visible, dimension within the neck. This might be due to an over or under energetic thyroid, iodine deficiency and also in rare situations thyroid cancer.


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Fig 3 – A thyroid goitre


Clinical relevance – Hyperthyroidism

Hyperthyroidism is the medical term because that an overactive thyroid gland. One common cause of Hyperthyroidism is Grave’s Disease – an autoimmune condition where antitoxin are created that wake up the TSH receptors on follicular cells. It affects about 1% the the population and is 10 times much more common in females than in men.

Patients may current with warmth intolerance, weight loss, tachycardia, nervousness, enhanced sweating, exophthalmos and also increased bowel movements. Hyperthyroidism have the right to be treated through Carbimazole which inhibits iodine binding to thyroglobulin.

Clinical relationship – Hypothyroidism

Hypothyroidism is one underactive thyroid gland. One common cause of Hypothyroidism is Hashimoto’s Disease – one autoimmune problem where thyroid follicles are ruined or antibodies are created that block the TSH receptor top top follicle cells.

Like hyperthyroidism, roughly 1% that the populace is affected with it being 10 times more common in women than in men. In the arising world, the many common reason of Hypothyroidism is iodine deficiency.

Patients can existing with cold intolerance, weight gain, bradycardia, negative concentration, myxoedema, dry skin, some hair loss and also constipation. Hypothyroidism deserve to be treated with oral T4 tablet computers (100-200 µg/day), to replace the hormone the is not being produced by the body.

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One way to remember the associated diseases through hyperthyroidism and also hypothyroidism is come look in ~ the influential vowels in each: hypErthyroidism is resulted in by gravE‘s disease, conversely, hypOthyroidism is resulted in by hashimOtO‘s disease.

Table 1 – Clinical features of hyperthyroidism and hypothyroidism
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