Anatomy of thyroid and parathyroid glands

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Anatomy of thyroid and parathyroid glands

  1. 1. Anatomy of thyroid and parathyroid glands BY- DR DHAVAL TRIVEDI
  2. 2.  THOMAS WARTHON named it after its shape of an ancient GRECIAN SHIELD IN 1656
  3. 3. developement  The thyroid gland develops from an endodermal extension in the floor of the pharynx known as the thyroglossal duct. The thyroglossal duct descends through the tongue and usually degenerates when the thyroid develops.  The vestigial marking of the thyroglossal duct is the foramen cecum of the tongue.  Remnants of thyroglossal duct anywhere else in its track can develop into a thyroglossal cyst. These present as neck lumps that rise when the tongue is protruded.  In 50% of people a remnant of this duct forms a small pyramidal lobe extending superiorly from the isthmus.  The parafollicular (C) cells are derived from the neural rest.
  4. 4.  The thyroid gland, brownish-red and highly vascular, is placed anteriorly in the lower neck, level with the fifth cervical to the first thoracic vertebrae  It is ensheathed by the pretracheal layer of deep cervical fascia and consists of right and left lobes connected by a narrow, median isthmus. Its weight is usually c.25g, but this varies.  The gland is slightly heavier in females, and enlarges during menstruation and pregnancy.  Estimation of the size of the thyroid gland is clinically important in the evaluation and management of thyroid disorders
  5. 5.  It is located in the neck, inferior to the larynx and cricoid cartilage.  The lobes of the thyroid gland are approximately conical.  Their ascending apices diverge laterally to the level of the oblique lines on the laminae of the thyroid cartilage, and their bases are level with the fourth or fifth tracheal cartilages.  Each lobe is c.5 cm long, its greatest transverse and anteroposterior extents being c.3 cm and 2 cm respectively.  The posteromedial aspects of the lobes are attached to the side of the cricoid cartilage by a lateral thyroid ligament
  6. 6.  The isthmus connects the lower parts of the two lobes, although occasionally it may be absent  It measures c.1.25 cm transversely and vertically, and is usually anterior to the second and third tracheal cartilages, though often higher or sometimes lower because its site and size vary greatly.
  7. 7.  A conical pyramidal lobe often ascends towards the hyoid bone from the isthmus or the adjacent part of either lobe (more often the left).  The thyroid gland is surrounded by a fibrous capsule derived from the pretracheal layer of the deep cervical fascia.  Extensions of this capsule into the body of the thyroid create septae, which divide the gland into lobules.  This connective tissue firmly connects the thyroid to the larynx and explains why the thyroid moves on swallowing.
  8. 8. SURFACES AND RELATIONS  The convex lateral (superficial) surface is covered by sternothyroid, whose attachment to the oblique thyroid line prevents the upper pole of the gland from extending on to thyrohyoid  More anteriorly lie sternohyoid and the superior belly of omohyoid, overlapped inferiorly by the anterior border of sternocleidomastoid  The medial surface of the gland is adapted to the larynx and trachea, contacting at its superior pole the inferior pharyngeal constrictor and the posterior part of cricothyroid, which separate it from the posterior part of the thyroid lamina and the side of the cricoid cartilage  The external laryngeal nerve is medial to this part of the gland as it passes to supply cricothyroid. Inferiorly, the trachea and, more posteriorly, the recurrent laryngeal nerve and oesophagus (which is closer on the left) are medial relations.  The posterolateral surface of the thyroid gland is close to the carotid sheath, and overlaps the common carotid artery
  9. 9.  The thin anterior border of the gland, near the anterior branch of the superior thyroid artery, slants down medially.  The rounded posterior border is related below to the inferior thyroid artery and its anastomosis with the posterior branch of the superior thyroid artery  The lower end of the posterior border on the left side lies near the thoracic duct  The isthmus is covered by sternothyoid, from which it is separated by pretracheal fascia. More superficially it is covered by sternohyoid, the anterior jugular veins, fascia and skin.  The superior thyroid arteries anastomose along its upper border and the inferior thyroid veins leave the gland at its lower border
  10. 10. Blood supply, nerves  The thyroid is highly vascular, and a bruit (the sound of turbulent blood flow) is sometimes heard in overactive glands  It is supplied by two arteries that anastomose within the gland: the inferior and superior thyroid arteries  The inferior thyroid artery is a branch of the thyrocervical trunk that arises from the subclavian arteries  It ascends behind the carotid sheath to enter the thyroid posteriorly
  11. 11.  The right recurrent laryngeal nerve is intimately related to this artery near the inferior pole of the thyroid gland. Surgery to the thyroid gland can damage this nerve, causing temporary difficulty with speaking.  To minimize the risk to this nerve, the artery is ligated far away from the thyroid gland during thyroidectomy.
  12. 12.  The superior thyroid artery is usually the first branch of the external carotid artery.  The external laryngeal nerve is related to this artery, but it is at less risk than the recurrent laryngeal during thyroid surgery.  The superior thyroid artery is ligated close to the thyroid gland to reduce this risk
  13. 13.  A third artery, called the thyroid ima artery, is present in 10% of people. It supplies the isthmus and it arises near the aortic arch, although the exact origin varies
  14. 14.  The thyroid gland is drained by three veins: • Superior thyroid vein. • Middle thyroid vein. • Inferior thyroid vein.  The first two veins drain into the internal jugular, whereas the inferior vein drains into the brachiocephalic veins.  Thyroid lymphatics drain into four groups of nodes: • Prelaryngeal lymph nodes. • Pretracheal lymph nodes. • Paratracheal lymph nodes. • Deep cervical lymph nodes
  15. 15. HORMONES  The thyroid gland synthesizes and secretes three hormones: • Thyroxine (T4). • Tri-iodothyronine (T3). • Calcitonin
  16. 16. Regulation  Hypothalamic thyrotrophin-releasing hormone (TRH) stimulates the release of thyroid-stimulating hormone (TSH) from thyrotrophs in the anterior pituitary gland and also causes upregulation of TSH gene transcription.
  17. 17. Thyroxine (T4) and Tri-iodothyronine (T3).  Both of these hormones profoundly increase the metabolic rate of the body.  Complete lack of thyroid secretion usually causes the basal metabolic rate to fall 40 to 50 per cent below normal, and extreme excesses of thyroid secretion can increase the basal metabolic rate to 60 to 100 per cent above normal.  Thyroid secretion is controlled primarily by thyroid- stimulating hormone (TSH) secreted by the anterior pituitary gland.
  18. 18.  About 93 per cent of the metabolically active hormones secreted by the thyroid gland is thyroxine, and 7 per cent triiodothyronine.  However, almost all the thyroxine is eventually converted to triiodothyronine in the tissues, so that both are functionally important.  The functions of these two hormones are qualitatively the same, but they differ in rapidity and intensity of action.  Triiodothyronine is about four times as potent as thyroxine, but it is present in the blood in much smaller quantities and persists for a much shorter time than does thyroxine
  19. 19.  The thyroid gland is composed of large numbers of closed follicles (100 to 300 micrometers in diameter) filled with a secretory substance called colloid and lined with cuboidal epithelial cells that secrete into the interior of the follicles.  The major constituent of colloid is the large glycoprotein thyroglobulin, which contains the thyroid hormones within its molecule.  Once the secretion has entered the follicles, it must be absorbed back through the follicular epithelium into the blood before it can function in the body.  The thyroid gland has a blood flow about five times the weight of the gland each minute, which is a blood supply as great as that of any other area of the body  To form normal quantities of thyroxine, about 50 milligrams of ingested iodine in the form of iodides are required each year, or about 1 mg/week.
  20. 20. Calcitonin  Calcitonin (also known as thyrocalcitonin) is a 32-amino acid linear polypeptide hormone  Calcitonin was purified in 1962 by Copp and Cheney.  While it was initially considered a secretion of the parathyroid glands it was later identified as the secretion of the C-cells of the thyroid gland  It acts to reduce blood calcium , opposing the effects of PTH - Inhibits Ca2+ absorption by the intestines - Inhibits osteoclast activity in bones - Inhibits renal tubular cell reabsorption of Ca2+ allowing it to be excreted in the urine  There is a relationship between the subcutaneous dose of calcitonin and peak plasma concentrations. Following parenteral administration of 100 IU calcitonin, peak plasma concentration lies between about 200 and 400 pg/ml. Higher blood levels may be associated with increased incidence of nausea, vomiting, and secretory diarrhea.
  21. 21. Physiologic Functions of the Thyroid Hormones Increase the Transcription of Large Numbers of Genes  Increase Cellular Metabolic Activity  Thyroid hormone has both general and specific effects on growth  Stimulation of Carbohydrate Metabolism  Stimulation of Fat Metabolism  Effect on Plasma and Liver Fats. - increased thyroid hormone decreases the concentrations of cholesterol, phospholipids, and triglycerides in the plasma, even though it increases the free fatty acids. -Conversely, decreased thyroid secretion greatly increases the plasma concentrations of cholesterol, phospholipids, and triglycerides and almost always causes excessive deposition of fat in the liver as well
  22. 22.  Increased Requirement for Vitamins. - Therefore, a relative vitamin deficiency can occur when excess thyroid hormone is secreted, unless at the same time increased quantities of vitamins are made available.  Increased Basal Metabolic Rate -Decreased Body Weight - Increased Blood Flow and Cardiac Output - Increased Heart Rate - Increased Respiration - Increased Gastrointestinal Motility  Slight increase in thyroid hormone usually makes the muscles react with vigor, but when the quantity of hormone becomes excessive, the muscles become weakened because of excess protein catabolism  Muscle Tremor  Effect on sleep - extreme somnolence is characteristic of hypothyroidism, with sleep sometimes lasting 12 to 14 hours a day.
  23. 23. Regulation of Thyroid Hormone Secretion  TSH, also known as thyrotropin, is an anterior pituitary hormone, a glycoprotein with a molecular weight of about 28,000.  It s effect on thyroid is Increased proteolysis of the thyroglobulin Increased activity of the iodide pump Increased iodination of tyrosine to form the thyroid hormones Increased size and increased secretory activity of the thyroid cells Increased number of thyroid cells  The most important early effect after administration of TSH is to initiate proteolysis of the thyroglobulin, which causes release of thyroxine and triiodothyronine into the blood within 30 minutes. The other effects require hours or even days and weeks to develop fully.
  24. 24.  Feedback Effect of Thyroid Hormone to Decrease Anterior Pituitary Secretion of TSH Increased thyroid hormone in the body fluids decreases secretion of TSH by the anterior pituitary. When the rate of thyroid hormone secretion rises to about 1.75 times normal, the rate of TSH secretion Falls essentially to zero
  25. 25.  Drugs that suppress thyroid secretion are called antithyroid substances.The best known of these substances are thiocyanate, propylthiouracil, and high concentrations of inorganic iodides.  Iodides in high concentrations decrease all phases of thyroid activity, they slightly decrease the size of the thyroid gland and especially decrease its blood supply, in contradistinction to the opposite effects caused by most of the other antithyroid agents.  For this reason, iodides are frequently administered to patients for 2 to 3 weeks before surgical removal of the thyroid gland to decrease the necessary amount of surgery, especially to decrease the amount of bleeding.
  26. 26. DISEASES OF THYROID  HYPERTHYROIDISM -excess of thyroid hormone production.  When this becomes symptomatic it is called thyrotoxicosis.  Thyrotoxicosis can occur in the absence of true hyperthyroidism.  It is a common disorder affecting 1/50 females and 1/250 males  Presentation is usually slow with a history lasting over 6 months.
  27. 27. SIGNS AND SYMPTOMS  Hair - loss  Eyes – exophthalmos,lid retraction,lid lag ,predisposes to keratitis  Brain - emotional lability,fatigue,anxiety,restlessness  Heart - palpitations,tachycardia ,atrial fibrillation  Neck – goitre  Muscles - proximal myopathy  Bowel - diarrhoea,increased appetite  Uterus - menorrhagia,infertility,reduced libido  Hands - tremor,warmth,sweating  Reflexes – increased  bones – osteoporosis  skin and adipose tissue - increased sweating,heat intolerance ,weight loss
  28. 28. The main causes of hyperthyroidism are:  Diffuse toxic goitre: Graves’ disease—an autoimmune disease involving autoantibody stimulation of TSH receptors.  Toxic multinodular goitre—nodular enlargement of the thyroid in the elderly.  Toxic nodule—autonomously functioning thyroid nodule; most are adenomas (benign thyroid hormone producing tumours).  Lymphocytic thyroiditis—inflammation causes release of stored hormones  Subacute thyroiditis—thyroiditis associated with a painful goitre.
  29. 29. Diagnosis  Thyroid function tests  Serum TSH, free T3, and free T4 are measured by radioimmunoassay - Raised T3 and T4 indicate that hyperthyroidism is present. Raised TSH suggests the fault lies in or above the pituitary gland, whereas low TSH points to a thyroid organ lesion  Autoantibody detection, e.g. Graves’ disease.  Radioisotope scanning to show the size of the thyroid gland and any abnormal ‘hot’ areas such as a toxic adenoma.  ECG for sinus tachycardia or atrial fibrillation
  30. 30. Treatment  Carbimazole—this drug inhibits the peroxidase reactions of T3 and T4 synthesis. It takes 3–4 weeks to have an effect  Radioactive iodine therapy—131I is only taken up by thyroid tissue; it kills the cells leading to reduced T3 and T4 synthesis. The response is slow and carbimazole may be required  Partial thyroidectomy—the thyroid gland is surgically removed leaving some tissue and the parathyroid glands. Used in patients with relapsing disease or allergy to medical treatment. Carries risk of recurrent laryngeal nerve palsy and hypocalcaemia due to the removal of parathyroid glands.
  31. 31. Hypothyroidism  Hypothyroidism is defined as an underactive thyroid gland leading to deficient thyroid hormones .  When this becomes symptomatic, it is called myxoedema. It is slightly less common than hyperthyroidism, affecting 1/100 females and 1/500 males  Presentation is even more gradual than in hyperthyroidism, with many symptoms frequently being ignored
  32. 32. Signs and symptoms  Hair - coarse and thin hair,loss of outer third of eyebrows  Brain- mental slowing,apathy,tiredness,psychosis  Face -pale puffy face, coarse features,deafness  Heart – bradycardia  Neck – goitre  Muscles - slowing of activity,muscle weakness in upper arms and legs  Bowel – constipation  Hands - cold hands,carpal tunnel syndrome  Uterus – amenorrhoea  Reflexes - slow relaxing  skin and adipose tissue - weight gain/obesity,intolerance to cold decreased sweating ,chronic oedema ,cold dry skin
  33. 33. Diagnosis  Hypothyroidism is not investigated as thoroughly as hyperthyroidism, since treatment does not vary.  Free T3 and T4 levels are low, whereas TSH levels are usually raised.  If TSH is low then a lesion of the hypothalamus or pituitary is likely.  Autoantibodies can be detected in Hashimoto’s thyroiditis
  34. 34. Treatment  All hypothyroidism is treated with thyroxine (T4) administered as an oral tablet in varying doses.  The dose is increased over several months, with regular monitoring of TSH levels until they are within the normal boundaries.  This process is slow, since it takes 4 weeks for TSH levels to reflect an increased dose due to the long half-life of thyroxine.  Thyroxine therapy is usually maintained for life
  35. 35. Hashimoto’s thyroiditis  When the thyroid gland is inflamed, the disease is called thyroiditis. This can be caused by autoimmune or viral processes.  Hashimoto’s thyroiditis is a destructive autoimmune disease that is especially common in middle-aged women.  It is mediated by autoantibodies against rough endoplasmic reticulum (microsomal antibodies) or thyroglobulin. The presence ofthese antibodies can be tested to confirm the diagnosis.  The thyroid gland is infiltrated by lymphocytes that cause the gland to enlarge, forming a goitre.  The initial destruction of the thyroid gland can release the thyroglobulin colloid causing temporary hyperthyroidism. The patients usually progress to a euthyroid (normal) state and finally develop progressive hypothyroidism.
  36. 36. Goitres  A goitre is a swelling in the neck caused by an enlarged thyroid gland. It is a common finding, and it is usually asymptomatic; however, large goitres can compress the oesophagus and trachea.  If a goitre is associated with hyperthyroidism it is described as ‘toxic’. Non-toxic goitres secrete normal or reduced levels of thyroid hormones.  Non-toxic goitres are usually the result of excessive TSH stimulation in the presence of hypothyroidism.  Goitres are treated by correcting the underlying pathology or by surgical removal for cosmetic reasons or to prevent compression of surrounding structures
  37. 37. Parathyroid glands  The parathyroid glands are small, yellowish-brown, ovoid or lentiform structures, usually lying between the posterior lobar borders of the thyroid gland and its capsule  They are commonly c.6 mm long, 3-4 mm across, and 1-2 mm from back to front, each weighing about 50 mg. Usually there are two on each side, superior and inferior.
  38. 38.  The superior parathyroid glands are more constant in location than the inferior and are usually to be found midway along the posterior borders of the thyroid gland, although they may be higher.  The inferior pair are more variably situated and may be within the fascial thyroid sheath, below the inferior thyroid arteries and near the inferior lobar poles; or outside the sheath, immediately above an inferior thyroid artery; or in the thyroid gland near its inferior pole.  The superior parathyroids are usually dorsal, the inferior parathyroids ventral, to the recurrent laryngeal nerves.  The parathyroid glands are very flattened in cross-section and are not normally visible by current imaging methods, including scintigraphy
  39. 39. VASCULAR SUPPLY AND LYMPHATIC DRAINAGE  The parathyroid glands have a rich blood supply from the inferior thyroid arteries or from anastomoses between the superior and inferior vessels.  Approximately one-third of human parathyroid glands have two or more parathyroid arteries.  Lymph vessels are numerous and associated with those of the thyroid and thymus glands.
  40. 40. INNERVATION  The nerve supply is sympathetic, either direct from the superior or middle cervical ganglia or via a plexus in the fascia on the posterior lobar aspects.  Parathyroid activity is controlled by variations in blood calcium level: it is inhibited by a rise and stimulated by a fall.  The nerves are believed to be vasomotor but not secretomotor
  41. 41. Parathyroid hormones  Parathyroid hormone (PTH), parathormone or parathyrin, is secreted by the chief cells of the parathyroid glands  It acts to increase the concentration of calcium (Ca2+) in the blood,  PTH half-life is approximately 4 minutes  PTH can be measured in the blood in several different forms: intact PTH; N-terminal PTH; mid-molecule PTH, and C-terminal PTH, and different tests are used in different clinical situations.  The average PTH level is 10-60 pg/ml.
  42. 42. Functions  Parathyroid hormone raises blood Ca by acting on 3 organs: - Bone: main effect- stimulates osteoclasts -> bone breaks down -> Ca released - Intestines: increases uptake of Ca from intestine - Kidney: stimulates reabsorption of Ca from the Ca in kidney tubules  PTH reduces the reabsorption of phosphate from the proximal tubule of the kidney which means more phosphate is excreted through the urine  PTH increases the activity of 1-α-hydroxylase enzyme, which converts 25- hydroxycholecalciferol to 1,25-dihydroxycholecalciferol, the active form of vitamin D.
  43. 43.  Stimulators - Decreased serum Ca2+ - Mild decreases in serum Mg2+ - An increase in serum phosphate  Inhibitors - Increased serum Ca2+ - Severe decreases in serum Mg2+, which also produces symptoms of hypoparathyroidism[
  44. 44. Clinical significance  Hyperparathyroidism, the presence in the blood of excessive amounts of parathyroid hormone, occurs in two very distinct sets of circumstances.  Primary hyperparathyroidism is due to autonomous, abnormal hypersecretion of PTH in the parathyroid gland while secondary hyperparathyroidism is an appropriately high PTH level seen as a physiological response to hypocalcaemia  A low level of PTH in the blood is known as hypoparathyroidism and is most commonly due to damage to or removal of parathyroid glands during thyroid surgery
  45. 45.  THANK YOU

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