Thyroid Gland | Thyroid Hormone Synthesis | Steps of T3 and T4 synthesis | Endocrine System Notes for NEET, GPAT, UPSC, SSC, NET
Thyroid Gland
The butterfly-shaped thyroid gland is located just inferior to the larynx (voice box). It is composed of right and left lateral lobes, one on either side of the trachea, that are connected by an isthmus anterior to the trachea . A small, pyramidal-shaped lobe sometimes extends upward from the isthmus. The normal mass of the thyroid is about 30 g (1 oz). It is highly vascularized and receives 80–120 mL of blood per minute.
Microscopic spherical sacs called thyroid follicles make up most of the thyroid gland. The wall of each follicle consists primarily of cells called follicular cells,
most of which extend to the lumen (internal space) of the follicle. A basement membrane surrounds each follicle.
When the follicular cells are inactive, their shape is low cuboidal to squamous, but under the influence of TSH they become active in secretion and range from cuboidal to low columnar in shape. The follicular cells produce two hormones: thyroxine , which is also called tetraiodothyronine or T4 because it contains four atoms of iodine, and triiodothyronine or T3, which contains three atoms of iodine.
T3 and T4 together are also known as thyroid hormones. A few cells called parafollicular cells or C cells lie between follicles. They produce the hormone calcitonin , which helps regulate calcium homeostasis.
Synthesis of Thyroid Hormone (T3 and T4)
The thyroid gland is the only endocrine gland that stores its secretory product in large quantities—normally about a 100-day supply. Synthesis and secretion of T3 and T4 occurs as follows
1. Iodide trapping: Thyroid follicular cells trap iodide ions (I-) by actively transporting them from the blood into the cytosol. As a result, the thyroid gland normally contains
most of the iodide in the body.
2 Synthesis of thyroglobulin: While the follicular cells are trapping I-, they are also synthesizing thyroglobulin (TGB), a large glycoprotein that is produced in the rough endoplasmic reticulum, modified in the Golgi complex, and packaged into secretory vesicles. The vesicles then undergo exocytosis, which releases TGB into the lumen of the follicle.
●3 Oxidation of iodide: Some of the amino acids in TGB are tyrosines that will become iodinated. However, negatively charged iodide ions cannot bind to tyrosine until
they undergo oxidation (removal of electrons) to iodine: I- to I2. As the iodide ions are being oxidized, they pass through the membrane into the lumen of the follicle.
●4 Iodination of tyrosine: As iodine molecules (I2) form, they react with tyrosines that are part of thyroglobulin molecules. Binding of one iodine atom yields monoiodotyrosine (T1), and a second iodination produces diiodotyrosine (T2). The TGB with attached iodine atoms, a sticky material that accumulates and is stored in the lumen of the thyroid follicle, is termed colloid.
●5 Coupling of T1 and T2: During the last step in the synthesis of thyroid hormone, two T2 molecules join to form T4 or one T1 and one T2 join to form T3.
●6 Pinocytosis and digestion of colloid: Droplets of colloid reenter follicular cells by pinocytosis and merge with lysosomes. Digestive enzymes in the lysosomes break down TGB, cleaving off molecules of T3 and T4.
●7 Secretion of thyroid hormones: Because T3 and T4 are lipid soluble, they diffuse through the plasma membrane into interstitial fluid and then into the blood. T4 normally is secreted in greater quantity than T3, but T3 is several times more potent. Moreover, after T4 enters a body cell, most of it is converted to T3 by removal of one iodine.
●8 Transport of T3 and T4 in the blood: More than 99% of both the T3 and the T4 combine with transport proteins in the blood, mainly thyroxine-binding globulin (TBG)
MCQ on Thyroid Hormone:
1. The main hormone secreted by the Thyroid gland
(a) T4
(b) T3
(c) (a) and (b) Both
(d) TSH
2. Which of the following is steps of synthesis of T3 and the T4
(a) Iodide trapping
(b) Synthesis of thyroglobulin
(c) Coupling of T1 and T2
(d) All
3. Which of the following hormone is involved in stimulation of Iodide trapping
(a) TRH
(b) TSH
(c) (a) and (b) Both
(d) None
4. Which of the following Transporter is involved in Iodide trapping
(a) Sodium Iodide Transporter
(b) Potassium Iodide Transporter
(c) Calcium Iodide Transporter
(d) None
5. Which of the following Transporter is involved putting iodide into lumen
(a) Pendrin
(b) Megalin
(c) Dendrin
(d) None
Correct Answer
1 -c
2 -d
3 -b
4 -a
5 -a