Thyroid gland |Endocrine gland |Hormone|Thyroxin|Functions|Physiology|Biology |

                  Endocrine|Thyroid gland

Thyroid gland|isthumus|thyos|size| Thyroxine |calcitonin| thyroglobulin |thyrocyte| pyramidal lobe' | Lalouette's lobe' | Berry's ligament.| transthyretin |thyroxin binding globulin|. parathyroid hormone |C cells| hormone receptors  TR-α₁ and α₂, TR-β₁ and β2|

The thyroid gland is a butterfly-like organ, an endocrine gland, present superficially in the neck. It consists of two lobes right lobe and the left lobe. In the lower two-thirds, the lobes are connected by a thin band of tissue known as the Isthmus.

Shape: It is a butterfly-like endocrine organ. In the 1600s, Thomas Wharton named this gland due to its shape resembling an ancient Greek shield or 'thyos.'

Size: The weight of the gland is 25 grams. Each lobe is 5 cm long,3 cm wide, and 2 cm thick. The Isthmus is 1.25 cm in height and breadth. The thyroid gland is more prominent in women than men and increases during pregnancy or any other condition with anxiety and tension. The thyroid gland is very vascular and the largest endocrine gland.

Table of contents

1.	Introduction
2.	Location Ø Size Ø Shape Ø Relations Ø Variations
3.	Development of the gland Ø Blood supply Ø Nerve supply Ø Venous drainage Ø Lymphatic drainage
4.	Iodine metabolism
5.	Secretion of hormones Ø MIT Ø DIT Ø T3 Ø T4.
6.	Mechanism of action
7.	Functions of the hormones

 Location: The thyroid gland is present in front of the larynx and trachea.

Relation of the thyroid gland :

Above: The thyroid and cricoid cartilages are above.

Below: The Adam's Apple is below the gland.

Front: 'Infra thyroid muscles' lie in front.

Behind: The larynx, lower pharynx, and esophagus lie behind.

On sides: The sternocleidomastoid muscles and carotid arteries on both sides.

Four parathyroid glands are present, two on each side. They are present between the two layers of the thyroid capsule at the back of the thyroid lobes.

A two-layered thin fibrous capsule covers the gland. The inner layer invaginates the gland mass and forms septums that divide the thyroid tissue into macroscopical lobes. The outer layer fuses below continues with the pretracheal fascia and forms 'a posterior suspensory ligament of the thyroid gland. The ligament is known as Berry's ligament.

Due to this ligament, the thyroid gland moves with the movement of these cartilages during swallowing.

                                               Blood supply

Ø Arterial blood supply is by Superior and inferior thyroid arteries.

Ø Venous blood drainage: Superior, middle, and inferior thyroid veins.

                                              Lymphatic drainage 

The lymph drains in prelaryngeal, pretracheal, and paratracheal lymph nodes.

                                             Nerve supply:

Ø Sympathetic nerve supply: Superior, middle, and inferior cervical ganglion of the sympathetic trunk.

Ø Parasympathetic nerve supply: Superior and recurrent laryngeal nerves.

                                            Variations

There are many variations. For example, sometimes, a third lobe is present, known as the 'pyramidal lobe' or 'Lalouette's pyramid.'

                                            Development of the gland 

At three weeks of gestation, the gland develops in the pharyngeal floor, at the base of the tongue. It then grows in front of the pharynx. In the next few weeks, it migrates to the bottom of the neck. The gland remains connected to the tongue during migration by a narrow tube known as a thyroglossal duct. At the end of the 5th week, the duct degenerates, and over the following two weeks, the detached gland migrates to its final position. In some cases, the thyroglossal duct remains patent even during an adult's life.

                                            Histology of thyroid gland:

The thyroid is made up of multiple small follicles. The size of a follicle is only 0.02 -0.9 mm. About 40 small follicles are grouped to form a lobule. A single layer of cuboidal cells is arranged around the lumen of a follicle. When the gland is active, cuboidal cells change into the columnar cells, and when the gland is inactive, the cuboidal cells become flat epithelial cells. The follicular cell is also known as the thyrocyte.

Some neuroendocrine parafollicular cells, also known as C cells, are present among the follicular cells and secrete 'calcitonin.'

                                            Secretion of the thyroid gland

The thyroid gland secretes three hormones :

(i) Thyroxine (T4)

(ii) Tri-iodothyronine (T3), and

(iii) Calcitonin.

The follicular lumen is filled with a clear gelatinous pink material - colloid containing Iodine known as Thyroglobulin.

The thyroid cells are very active and have many mitochondria, rough endoplasmic reticulum, and Golgi apparatus. Their microvilli project in the lumen of the follicle. They synthesize protein and secrete it into the lumen by cytosis. The rough endoplasmic reticulum of thyrocytes synthesized Thyroglobulin. Thyroglobulin enters the thyroid follicles lumen by exocytosis. It is stored as a colloid and reacts with iodine in the follicular lumen.

                                                        Iodine metabolism

Iodine is a trace element and essential for thyroid hormone production. In blood, iodide is( I-) present. Thyrocytes absorb iodides against the electrical gradient by an 'ion channel.' In the sodium-iodide symporter channel on the cell membrane, two sodium ions move from the cell, and one iodide ion moves into the cell. Finally, the iodide passes from the cell into the follicular lumen through the action of 'pendrin' an 'iodide-chloride antiporter.'

In the follicular lumen, iodide is oxidized to iodine and becomes very active. In the presence of an enzyme' thyroid peroxidase', it combines with active tyrosine of the Thyroglobulin to form MIT and DIT.

At first, MIT monoiodotyrosine is formed, then diiodotyrosine DIT.

MIT+MIT=DIT

DIT+MIT=Triiodotyrosine T3

DIT+DIT=Tetraiodotyrosine T4.

The follicular cells reabsorb Thyroglobulin from the lumen. The iodinated tyrosines are cleaved and form MIT, DIT, T3, T4.and' reverse triiodothyronine.'

T3 and T4 enter the blood. In blood, 80-90% is T4 and 20-30% T3.

MIT contains one atom of iodine per molecule, DIT contains two iodine atoms per molecule, T3 contains three iodine atoms per molecule, and T4 contains four iodine atoms per molecule.

Only small amounts of thyroid hormones, 0.3%, are free and active. T4 in the circulation produces 85% of T3.

70% are bound to thyroxin binding globulin.

15% are bound to albumin, and

10% are bound to transthyretin.

C cells present between the follicular cells secrete Calcitonin which regulates blood calcium concentration. Calcitonin decreases calcium release from the bone and increases its absorption when the calcium level rises in the blood. Calcitonin helps to regulate blood calcium levels.

1. Reduce the release of calcium from bone.

2. Increases calcium absorption by bone.

Calcitonin is not as potent as the parathyroid hormone secreted by the gland and opposes the effect of the parathyroid hormone.

                                                Mechanism of action

Thyroid hormones enter the cell by crossing the cell membrane and binding to intracellular nuclear thyroid hormone receptor intracellular nuclear thyroid hormone receptors TR-α1 and α2, TR-β1 and β2.

These receptors bind with 'hormone response elements' and 'transcription factors to modulate DNA transcription.

In addition, thyroid hormones act with various enzymes in the cytoplasm, including calcium ATPase, adenylyl cyclase, and glucose transporters.

                                                Functions of the thyroid gland

Hormones of the thyroid gland are not essential for life, but their removal in adults leads to poor temperature regulation and mental and physical unhealthiness.

The thyroid hormones have many functions and affect all body tissues. These include

The thyroid hormones are required especially for normal fetal CNS development and fetal bone maturation.

The thyroid hormones

Ø Increase the 'basal metabolic rate 'and protein synthesis.

Ø Stimulating oxygen consumption of most body tissue helps regulate fat and carbohydrate metabolism and is essential for tissue differentiation.

Ø It stimulates absorption from the gut, uptake by cells, breakdown of glucose, breakdown of fats, increases the number of free fatty acids thyroid hormones, and decreases cholesterol levels.

Ø Cardiovascular effect of increasing the rate and strength of the heartbeat increases the rate of breathing intake and consumption of oxygen.

Ø The thyroid hormones are essential for maintaining normal sexual functions, including libido and a regular menstrual cycle.

Ø During development, deficiency of thyroid hormones in children will cause mental retardation.

Uthyroid is a state of normal thyroid functions.

Hyperthyroidism: when there is an excess of thyroid hormones.

Hypothyroidism: when there is a low amount of thyroid hormone.

Thyroiditis: when there is inflammation of the thyroid gland.

Goiter: when there is an enlargement of the thyroid gland.

Other conditions related to the thyroid glands are :

Thyroid nodules, thyroid cancer, autoimmune disorders, Grave disease, etc.

The gland and its various diseases have been noted and treated for centuries.

                                            How are these hormones metabolized?

Deiodinase enzymes in peripheral tissue remove the iodine from MIT and DIT and convert T4 to T3 and RT3.

The thyroid hormones have negative feedback on the TSH and CRH. A high level of thyroid hormones suppresses TSH production.

Hashtag: thyroid gland|isthumus|thyos|size| Thyroxine |calcitonin| thyroglobulin |thyrocyte| pyramidal lobe’ |Lalouette’s lobe’| Berry’s ligament.| transthyretin |thyroxin binding globulin|. parathyroid hormone |C cells| hormone receptors  TR-α₁ and α₂, TR-β₁ and β2

 Internal link: https://blog.totalphysiology.com/2022/01/pituitary-glandsecretionhormones.html

https://blog.totalphysiology.com/2021/03/2021-google-ductlessglands-we-must-know.html

https://blog.totalphysiology.com/2021/03/gastrointestinal-hormones-you-must-know.html

External link:  https://en.m.wikipedia.org>thyroid