Thyroid gland: A reddish-brown gland, it is the largest endocrine gland in the body and the only extracellular gland that can store a large amount of hormones produced.
It is located below the thyroid cartilage, on both sides of the trachea, and is connected by an isthmus in the middle. It is shaped like an "H", brownish-red, and has two lateral lobes.
The lobes on both sides are attached to the lateral surface of the lower larynx and the upper part of the trachea, up to the middle of the thyroid cartilage, down to the sixth tracheal cartilage, and the isthmus is mostly located in front of the second to fourth tracheal cartilage, and some people are underdeveloped. Sometimes a cone-shaped lobe sticks upwards from the isthmus, of varying lengths, and the elder can reach the hyoid bone, which is a remnant of embryonic development, and often gradually degenerates with age, so children are more common than adults.
The thyroid gland is surrounded by two layers of membranes: the inner layer of the propria and the outer layer of the surgical membrane.
The thyroid gland is surgically held to the trachea and cricoid cartilage, and the glands are suspended from the cricoid cartilage by suspensory ligaments medial to the upper poles of the left and right lobes, so that the glands move up and down during swallowing.
Artery: Superior thyroid artery;Inferior thyroid artery;The lowest artery of the thyroid gland;Vein: suprathyroid vein;Middle thyroid artery;Inferior thyroid vein;Nerve: superior laryngeal nerve;Recurrent laryngeal nerve .
The superior laryngeal nerve comes from the vagus nerve and accompanies the superior thyroid artery, which is divided into the inner and outer branches at the upper pole of the thyroid glandThe medial branch is the sensory branch, which is distributed on the laryngeal mucosa, and the outer branch is the motor branch, which innervates the cricothyroid muscle and tightens the vocal cords. The recurrent laryngeal nerve comes from the vagus nerve and walks in the groove between the trachea and esophagus, and mostly passes between the branches of the inferior thyroid artery, innervating vocal cord movementInjuries on one side can cause hoarseness, while injuries on both sides can cause vocal cord paralysis.
The basic unit of the thyroid gland is the acinar, which has a strong aggregation effect on iodine, although usually the iodine content in the gland is 25 50 times higher than the content in the blood, but the daily dietary intake of iodine still enters the thyroid gland 1 3, and 90% of the iodine content of the whole body is concentrated in the thyroid gland.
The process of TH synthesis by thyroid follicular epithelial cells: polyiodine Activation of iodine Iodine of tyrosine The condensation of iodine and thyroglobulin (TG) of iodide tyrosine are essential raw materials for TH synthesis.
Thyroid peroxidase (TPO) is a key enzyme in Th synthesis.
Thyroid follicular epithelial cells are the functional units that synthesize and secrete TH and are regulated by adenopituitary thyroid-stimulating hormone (TSH).
1. Polyiodine. The concentration of I- in the thyroid follicular epithelial cells is about 30 times the concentration of serum I-, and the process of iodine uptake by follicular epithelial cells is an active transport process carried out by a reverse electro-chemical gradient, which is called iodide trap, which belongs to secondary active transport, which is mediated by the sodium-iodide symporter (NIS) located in the basement membrane of follicular epithelial cells, and relies on the potential energy provided by sodium pump activity, and NIS can be 1i-:2Na+In clinical practice, radioactive iodine tracer is commonly used to check the polyiodine capacity and functional status of the thyroid gland, and the ability to take up iodine is enhanced when the thyroid is hyperactive, and the opposite is true when the function is reduced.
2. Activation of iodine.
Location: The junction between the microvilli of the membrane at the tip of the follicular epithelial cell and the follicular cavity (rich in peroxidase (TPO)) In the presence of H202, the inorganic iodine I- accumulated in the cell is activated into organic iodine I0 under the action of TPO.
3. Iodide of tyrosine.
The hydrogen on the tyrosine residue benzene ring on the TG molecule is replaced by activated iodine catalyzed by TPO if only H+ at the 3rd position of the benzene ring is replaced, and if only H+ at the 3rd position of the benzene ring is replaced, monoiodotyrosine (MIT) is generated, and if H+ is substituted at the 3,5th position of the benzene ring, diiodotyrosine (DIT) is generated
4. Condensation. Condensation, or coupling under the catalysis of TPO, the condensation of two DIT to form T4MIT and DIT condensation to form T3 and a very small amount of RT3 The proportion of organic iodide in the thyroid gland of normal adults is about 23 for MIT, about 33 for DIT, about 7 for T3, about 35 for T4, and about 1 for RT3 and other components. This ratio can be affected by iodine content, when the iodine content of the thyroid gland increases, the DIT increases, and the T4 content increases accordinglyIn iodine deficiency, MIT increases and T3 content increases.
Thyroid hormone (TH) is an iodide of tyrosine, including tetraiodothyronine (3,5,3).' ,5'traiodothyonine (T4 or thyroxine), triiodothyronine (3, 5, 3'tiodotoyonine, t) and very small amounts of inverse triiodothyronine (3, 3' ,5'tioiotonine, RT), which account for about 90%, 9%, and 1% of total secretions, respectively. Although the secretion of T4 is the largest, T is the most biologically active, about 5 times that of T4, and Rt does not have Th biological activity. Secretion of thyroid hormones
The human body produces 80 100 ug d of T4 (all produced by the thyroid gland) and 20 30 g of T per day (only 20% of T is produced by the thyroid gland, and the other 80% of T is converted by T4 of peripheral tissues).
After TH synthesis, it is generally stored in the follicular cavity of the thyroid gland in the form of colloid, and the reserve can ensure the metabolic needs of the body for up to 50 120 days.
Therefore, when antithyroid drugs** are used in clinical practice, it takes a long time for the drug to be effective.
The secretion of TH is controlled by TSH.
Under the action of TSH, the apical membrane microvilli of thyroid follicular epithelial cells extend out of pseudopods, and TG-containing glial droplets are ingested into the cells by swallowing.
The glial droplets then fuse with lysosomes to form phagosomes, and under the action of lysosomal proteases, the peptide bonds on the TG molecule are hydrolyzed, and free T4, T, MIT and DIT are released. MIT and DIT entering the cytosol are rapidly deiodized under the action of deiodinase, and most of the iodine removed can be reused.
T4 and T that enter the cytoplasm are insensitive to deiodinase in follicular epithelial cells and can be rapidly secreted from the bottom of the cell into the circulating bloodstream.
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