Thyroid Gland
· It is the largest endocrine gland, 25 g weight.
· Location:
· It is located in front of the lower half of the larynx and upper five tracheal rings at the level of the fifth cervical vertebra (C5) to first thoracic (T1) vertebra.
· It consists of two symmetrical lobes connected by an isthmus.
· It consists of two symmetrical lobes connected by an isthmus.
· These lobes are surrounded by the connective tissue capsule.
· It is endodermal in origin.
· Histologically made up of a large number of follicles (about five million follicles).
· It is endodermal in origin.
· Histologically made up of a large number of follicles (about five million follicles).
· Follicles are internally lined by simple cuboidal epithelium or principle cells.
· In between the follicles: Parafollicular or C-cells are present.
· The follicular lumen is filled by a proteinous substance called colloid.
· Colloid is thyroglobulin or glycoprotein with the molecular weight of 660000 containing 140 tyrosine residues.
· It is supplied by 2 pairs of thyrocervical arteries, 3 pairs of veins.
· It is supplied by 2 pairs of thyrocervical arteries, 3 pairs of veins.
· The blood flow is very high which is about 4-6ml/g/minute.
· The principal cells manufacture the thyroid hormones namely tetraiodothyronine (T4) and tri-iodothyronine (T3).
· Thyroxine (T4) is synthesized from iodine and an amino acid called tyrosine.
· About 90% of the secreted hormones are T4 and about 9% is T3.
· The principal cells manufacture the thyroid hormones namely tetraiodothyronine (T4) and tri-iodothyronine (T3).
· Thyroxine (T4) is synthesized from iodine and an amino acid called tyrosine.
· About 90% of the secreted hormones are T4 and about 9% is T3.
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| Structure of thyroid gland |
Synthesis of Thyroid Hormone:
· Absorption of dietary iodide - First the dietary iodine is converted into iodides which are absorbed from the upper parts of the intestine into the bloodstream.
· Iodine trapping - Then one-fifth of the iodide is trapped by the thyroid gland and used for the biosynthesis of thyroid hormone.
· The basal membrane of the thyroid follicle cells actively transport the iodide into the interior of the cells called iodide trapping.
· It is an active process and requires ATP. The concentration of iodine is thirty times more than blood. This process is supported by TSH.
· Oxidation of iodine - Then iodide is oxidized to the active stage of iodide by hydrogen peroxide and thyroid peroxidase.
· Organification - Active iodide is combined with tyrosine residue to form thyroglobulin molecules. The combining of thyroglobulin molecule with the iodine is called organification or iodination.
· Couping and storage - Then monoiodotyrosine (MIT) and di-iodotyrosine (DIT) are formed. Two adjacent DIT combines to form the major product of the thyroid gland or thyroxine. One molecule of MIT combined with 2 molecules of DIT to form triiodothyronine.
· Release of thyroid hormone - Finally thyroxine is released in the bloodstream.
· Regulation of secretion - TSH is the main regulator of thyroid gland functions. TSH acts directly on the thyroid gland and stimulates the secretion of thyroid hormone by – activation of iodide trapping and stimulation of every step in the synthesis and release of the hormone.
· The greatest concentration of iodine is found in the thyroid.
· The thyroid is the only endocrine gland that stores its secretory product in large quantities.
· Kendall (1914) was the first to prepare crystals of thyroxine.
· Harrington and Barger (1927) worked out the molecular structure of thyroxine.
· Secretion of thyroxine is inversely proportional to the blood level of thyroxine (feedback mechanism).
· Thyroid hormones enter the cytoplasm and bind to receptors in the nucleus to activate specific genes.
· Thyroid hormones also bind to receptors on mitochondria and accelerate ATP production.
Functions of Thyroid Gland:
· Thyroxine's main function is to control metabolism.
· Thyroid regulates the basal metabolic rate. ,
· Thyroid increases catabolism, produces energy and increases the body temperature. This process is called the calorigenic effect.
·Thyroxine also helps to regulate tissue growth and development,
· Thyroid initiates, regulates and plays a key role in .the metamorphosis of frog's tadpole.
· Gudernatsch (1912) reported the role of the thyroid in the metamorphosis of Amphibia.
· Hyposecretion of thyroxine retards and hypersecretion enhances the rate of metamorphosis.
· Addition of thyroxine or iodine in pond water induces and enhances metamorphosis in the tadpoles.
· On removing the thyroid gland from the tadpole, it will remain tadpole, throughout life.
a. Cretinism:
· Hyposecretion of thyroxine during the growth years or birth.
· It is called childhood hypothyroidism.
· Two important symptoms are dwarfism and severe mental retardation, failure of sexual development, potbelly and enlarged protruded tongue, bradycardia, weight gain etc.
b. Myxodedema (Gull's disease):
· It is in adulthood, hypothyroidism.
· Lack of thyroxine causes the body to retain water.
· The puffy appearance of the face.
· Patient suffers from slow heart rate, low body temperature, muscular weakness and oedema (accumulation of interstitial fluid that causes the facial tissues to swell and look fluffy).
c. Simple Goitre:
· It is caused by a lower intake of iodine through diet.
· Goitre is the swelling of the neck due to enlargement of the thyroid.
· Table salt is often iodised for certain areas to prevent goitre.
· Goitre is the swelling of the neck due to enlargement of the thyroid.
· Table salt is often iodised for certain areas to prevent goitre.
d. Exophthalmic Goitre or Basedow disease or Parry's disease:
· Also called Graves disease (named after the Irish physician Robert James Graves, 1796–1853) is hyperthyroidism.
· Eyeballs protrude due to the accumulation of mucus in eye orbits; the metabolic rate is abnormally high.
· Thyroid also contains parafollicular cells or C-cells that lie sandwiched between the cuboidal follicle cells and their basement membrane.
· The parafollicular cells secrete a mono-iodised hormone called 'Thyro-Calci-Tonin' (TCT) or calcitonin.
· Calcitonin lowers the amount of calcium and phosphate in the blood.
· Calcitonin retards bone dissolution (osteoclastic action) and stimulates excretion of calcium in the urine.
· Calcitonin lowers calcium in the Extra-Cellular Fluid (ECF).
· Thyroid also contains parafollicular cells or C-cells that lie sandwiched between the cuboidal follicle cells and their basement membrane.
· The parafollicular cells secrete a mono-iodised hormone called 'Thyro-Calci-Tonin' (TCT) or calcitonin.
· Calcitonin lowers the amount of calcium and phosphate in the blood.
· Calcitonin retards bone dissolution (osteoclastic action) and stimulates excretion of calcium in the urine.
· Calcitonin lowers calcium in the Extra-Cellular Fluid (ECF).
e. Hashimoto's disease:
· It is an autoimmune thyroid disorder discovered by Japanese surgeon Hakaru Hashimoto.
· It is characterized by the production of antibodies in response to thyroid antigens.
· It is also known as the suicide of the thyroid.
· The thyroid disorders are more frequent in females than in males.
Abnormal secretion of Thyroid Hormones
| Hyperthyroidism | Hypothyroidism |
|---|---|
| Increased BMR | Decreased BMR |
| Weight loss, increased appetite | Weight gain, anorexia |
| Anxiety, restlessness, excitability | Depression, psychosis, lethargy, mental slowness |
| Hair loss | Bradycardia |
| Tachycardia, palpitation, atrial fibrillation | Bradycardia |
| Diarrhoea | Constipation |
| Warm sweaty skin, heat intolerance | Dry cold skin, prone to hypothermia |
| Exophthalmos in Grave's disease. Toxic nodular goitre. | Cretinism in children, Myxoedema in adults. Autoimmune thyroiditis (Hashimoto's disease) |
Parathyroid Glands
a. Location and Shape:· Parathyroid glands are found embedded in the posterior surface of the thyroid which are four small oval bodies (6x3x2mm).
· One pair is arranged vertically behind each lobe.
b. Weight:
· About 50 mg (each), total-140mg.
· Histologically, they are formed by masses or columns of epithelial cells with large blood sinuses in between them.
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| Structure of parathyroid glands |
· The glandular cells are differentiated into two types.
(I) Chief cells or small cells with clear cytoplasm and non-granular. It is chief secretary cells.
(II) Oxyophil cells or Eosinophilic cells with Acidophilic cytoplasmic granules.
Two hormones are secreted by these glands:
1. Parathormone hormone (PTH) or Collip's Hormone:
1. Parathormone hormone (PTH) or Collip's Hormone:
2. Calcitonin
1. Parathormone hormone (PTH) or Collip's Hormone:
· It is a linear polypeptide with a molecular wt of 9500.
· It is made up of 84 aminoacids residues.
Regulation of Secretion:
· Serum ionized calcium level is the main regulator of PTH secretion by the negative feedback mechanism.
· Serum ionized calcium level is the main regulator of PTH secretion by the negative feedback mechanism.
· When the Calcium level is low (1.3 mv), the PTH synthesis and secretion is stimulated whereas increases in calcium level in the blood decrease the secretion of PTH.
· Functions of Parathromone:
· It increases the calcium level (hypocalcemic factor) and decreases the phosphate level in the blood plasma.
· It maintains the proper balance between Na+ and K+ on one hand and on the other hand Ca+ and Mg+ on other hand.
· It enhances the calcium absorption by the gut and glomerular filtration rate with Vitamin D.
· It enhances the calcium absorption by the gut and glomerular filtration rate with Vitamin D.
· It increases the level of serum and total Cholesterol.
· It affects the growth of bones by calcium mobilization from bone and helps in the osteoclastic process of bone or osteolysis, membrane permeability, nerves functioning and muscular activity of the body.
· It also increases the renal calcium reabsorption whereas decreases the renal phosphate and bicarbonates reabsorption.
· It affects the growth of bones by calcium mobilization from bone and helps in the osteoclastic process of bone or osteolysis, membrane permeability, nerves functioning and muscular activity of the body.
· It also increases the renal calcium reabsorption whereas decreases the renal phosphate and bicarbonates reabsorption.
· Disorders:
a. Hyperparathyroidism
· When the gland secretes a hyper amount of Parathromone, the level of calcium in the blood is increased due to decalcification.
a. Hyperparathyroidism
· When the gland secretes a hyper amount of Parathromone, the level of calcium in the blood is increased due to decalcification.
· It causes generalized Osteitis fibrosa cystica or osteoporosis. This disease is generally found between the age of 20-40 years.
· The patient suffers from weakness, loss of muscular tones, vomiting, reduces appetite, kidney stone due to Ca ions deposition, soft and porous bone due to decalcification and many bone cysts are formed.
· Ca+ deposition in lungs causing dyspnoea, may deposit in arteries raising arterial blood pressure.
· Remedy: Surgical removal of hyperactive parathyroid tissue.
· The patient suffers from weakness, loss of muscular tones, vomiting, reduces appetite, kidney stone due to Ca ions deposition, soft and porous bone due to decalcification and many bone cysts are formed.
· Ca+ deposition in lungs causing dyspnoea, may deposit in arteries raising arterial blood pressure.
· Remedy: Surgical removal of hyperactive parathyroid tissue.
b. Hypoparathyroidism
· Due to extirpation or hyposecretion, results in low blood calcium level and consequent increase in blood phosphate concentration which causes parathyroid tetany (neuromuscular hyperirritability).
· It is characterized by carpopedal spasm (permanent contraction of muscles of hand and feet), laryngeal stridor (sudden contraction of the laryngeal muscle which causes obstruction in respiration and patient may die) and Chvostek's sign (tapping over the facial nerves produces facial muscle spasms)
· Remedy: The normal level of blood calcium should be 10mg per 100ml of blood.
· Remedy: The normal level of blood calcium should be 10mg per 100ml of blood.
· If it is below 7mg per 100ml of blood the condition is called hypoparathyroidism.
2. Calcitonin:
· It is a peptide hormone of molecular weight 3400 consisting of 32 amino acids.
· Its secretion also depends on the concentration of calcium in the blood.
· The synthesis and secretion of calcitonin are stimulated by gastrin CCK, glucagon etc.
· Functions:
· Osteoclasts are the target cells of calcitonin decreasing their differentiation and activity.
· It decreases the bone resorption and level of calcium and phosphate in the blood.
· In the kidney, it increases the calcium, phosphate, sodium, potassium excretion or renal hypocalcemic effects.
· It is also used to treat Paget's disease (bone formation is very high, the patient suffers from deformities and pain) so calcitonin reduces the bone formation and pain.
· It is a peptide hormone of molecular weight 3400 consisting of 32 amino acids.
· Its secretion also depends on the concentration of calcium in the blood.
· The synthesis and secretion of calcitonin are stimulated by gastrin CCK, glucagon etc.
· Functions:
· Osteoclasts are the target cells of calcitonin decreasing their differentiation and activity.
· It decreases the bone resorption and level of calcium and phosphate in the blood.
· In the kidney, it increases the calcium, phosphate, sodium, potassium excretion or renal hypocalcemic effects.
· It is also used to treat Paget's disease (bone formation is very high, the patient suffers from deformities and pain) so calcitonin reduces the bone formation and pain.
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