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THYROID GLAND Captain Rishi Pokhrel
IntroductionUnique endocrine gland Located superficially Uses raw material – supplied externally ( Iodine ) Stores the product (2 months) Rich blood supply 5 ml/g/min 5 l/hr. 0.4% of body weight - 2% of total blood flow
Introduction – Historical background Eponymy – Gr. thyreos (Shield) Goiters were known long before the thyroid gland itself. God Bes of ancient Egypt – features of myxedema China 2700 B C Ayurveda 1400 BC – “galaganda”
Hippocrates (460-337 BC) “...when glands of the neck become diseased themselves, they become tubercular and produce struma....” (struma – goiter) Hippocrates failed to differentiate between the thyroid and the cervical glands
Gallen (130-200 AD) described operations on two boys by ignorant physicians who removed tubercular nodes with their fingernails, rendering one boy mute and the other semi- mute.secretions of the thyroid lubricated the larynx & cartilage ; aphonia was provoked by cutting the laryngeal nerves
Aetios 550 AD : goiter -> aneurysmBronchocele, elephantias is of the throat etc.
• Leonardo Da Vinci is generally credited as the first to draw the thyroid gland as an anatomical organ in 1508 AD
Andreas Vesalius (1514- 1564) correctly described the anatomy of thyroid gland in detailB. Eustachius (1520-1547) first used the term isthmusThomas Warton (1614- 1673) gave the gland its modern name of thyroid
Robert James Graves, 1835 – hyperthyroidism – Grave’s disease Partial thyroidectomy - P.S. Dessault (1744- 1795) in Paris. Guillance Dupuytren 1808 - total thyroidectomy for tumor Ludwig Rehn, 1880, first successful thyroidectomy for exophthalmic goiter. Thyroxine was identified only in the 19th century In 1909, Theodor Kocher won Nobel Prize in Medicine "for his work on the physiology, pathology and surgery of the thyroid gland”
Development Starts from 3rd week of IUL -1st endocrine gland to develop Proliferation of cells from caudal end of Thyroglossal duct - endoderm PF or C cells – Ultimibranchial body – 4th/5th pharyngeal pouch – neural crest cells
Week 3 (day 24) appears as midline vesicular structure at foramen cecum form a duct like invagination of ventral pharyngeal endoderm grows caudally to become thyroglossal duct
Week 7 finishes descent along midline – forms median isthmus & 2 lateral lobes 2 lateral anlagen develop from 4th-5th branchial pouch, which contains ultimobranchial body midline and lateral portions of thyroid fuse Thyroglossal duct disappears – remnants: Pyramidal lobe (50%) and levator muscles
Week 9: cords and plates of follicular cells are formed Week 10:cords divide into small cellular groups, small follicular lumina appear Week 11-12: colloid secretion appears, thyroid becomes functional Week 14: well developed follicles are lined by follicular cells and contain thyroglobulin containing colloid in lumina Week 20: levels of TSH and T4 starts rising Week 35: TSH & T4 levels = adults Early growth and development is independent of TSH
Features Features Fleshy mass in the neck, in front of trachea, concealed by strap muscles of neck 2 symmetrical lobes united at isthmus. Lobes 5 x 3 X 2 cm; isthmus 1.25 x 1.25 cm 25 – 30 gms in wt. – variable, larger in females, varies with menstruation and pregnancy
Features Lobes – Pear shaped, triangular in cross section apex: oblique line of thyroid cartilage base: 4-6 tracheal ring Isthmus flat and square: against 2-4 tracheal rings Pyramidal lobe (50%) Levator Glandulae thyroidae
Coverings Inner true capsule: condensation of parenchyma Outer false capsule: formed by splitting of pretracheal layer of deep Cx fascia. Blood vessels ramify under true capsule Ligament of berry – condensation of PTF from false capsule to cricoid cartilage- RLN runs in it -> movement of thyroid gland with larynx
Parenchyma Follicles: arrangement of cells in hollow spherical or short cylindrical masses 0.2- 0.9 mm - Structural & functional units Filled with gel like substance - colloid- Thyroglobulin Simple Principal/Follicular cells Parafollicular or ‘C’ cells
Thyroglobulin - Stored follicle – iodine trapping and iodination - reuptake (Scalloped margins) – lysosmes - broken into T3 & T4 - secreted
(calcitonin)– Lie beside follicle– Enclosed in same BM but not reaching lumen– Larger, rounded & paler– Nucleus round /oval, eccentric– Secretory granules – Calcitonin (PTH Antagonist)
Phylogeny• Thyroid gland evolution -> adapt to the terrestrial ecosystem with less supply of iodine.• Jellyfish lack thyroid gland• Endostyle of non-vertebrate chordates -> homologous to thyroid (Endostyle: longitudinal ciliated groove on ventral wall of the pharynx – produces mucus to gather food particles)• In lampreys, the larval endostyle transforms into adult thyroid gland during metamorphosis• Most primitive vertebrates - follicular thyroid gland but non capsulated• Thyroid is encapsulated in cartilaginous fish• In the higher vertebrate forms, the thyroid is a one- or two-lobed encapsulated structure.
Thyroid hormones Primary function of the thyroid - production of T3, T4, and calcitonin T3 & T4 – essential for normal growth, development & metabolism T4 -> T3 by peripheral organs like liver, kidney, spleen T3 is 4 - 10 X more active than T4 Hypothalamo – pitutary – thyroid axis
Physiology Thyrocytes (follicular cells) have four functions: – collect and transport iodine – they synthesize thyroglobulin and secrete it into the colloid – fix iodine to the thyroglobulin to generate thyroid hormones – remove the thyroid hormones from thyroglobulin and secrete them into the circulation.
Thyroglobulin is synthesized in the rough endoplasmic reticulum and follows the secretory pathway to enter the colloid in the lumen of the thyroid follicle by exocytosis. Meanwhile, a sodium-iodide (Na/I) symporter pumps iodide (I-) actively into the cell, which previously has crossed the endothelium by largely unknown mechanisms. This iodide enters the follicular lumen from the cytoplasm by the transporter pendrin, in a purportedly passive manner In the colloid, iodide (I-) is oxidized to iodine (I0) by an enzyme called thyroid peroxidase. Iodine (I0) is very reactive and iodinates the thyroglobulin at tyrosyl residues in its protein chain (in total containing approximately 120 tyrosyl residues). In conjugation, adjacent tyrosyl residues are paired together. The entire complex re-enters the follicular cell by endocytosis. Proteolysis by various proteases liberates thyroxine and triiodothyronine molecules, which enters the blood by largely unknown mechanisms.
Calcitonin• 32 - aa linear polypeptide - C cells• Not under control of hypothalamus or pitutary• Secretion -> Ca2+, gastrin and pentagastrin• not essential for life – no replacement required following thyroidectomy unlike parathyroids.• antagonist to PTH - reduces Ca2+ level• Inhibits: Ca2+ absorption by intestine, osteoclast activity in bone & renal tubular cell reabsorption of Ca2+• Agonist to PTH -> Inhibits phosphate reabsorption by the kidney• Used clinically for Tt of hypercalcemia & osteoporosis
Applied AnatomyCongenital thyroid disorders Aberrant thyroid tissue Lingual thyroid Thyroglossal cyst 50% close to or just inferior to body of hyoid bone Thyroglossal fistula – secondary to rupture of cyst
Hyperthyroidism Vs. thyrotoxicosis Graves’ disease—an autoimmune disease involving autoantibody stimulation of TSH receptors. Toxic multinodular goiter — nodular enlargement of the thyroid in the elderly. Toxic nodule—autonomously functioning thyroid nodule; most are adenomas Lymphocytic thyroiditis /Hashimoto’s thyroiditis—inflammation causes release of stored hormones (followed by hypothyroid phase). Subacute (de Quervain’s) thyroiditis — thyroiditis associated with a painful goiter.
Applied anatomyThyroidectomy lobe, subtotal, total Transverse skin incision 2.5 cm above jugular notch Gap b/w ST & SH opened up – trachea & isthmus exposed Muscles retracted laterally or divided at upper ends – preserve nerve supply from ansa cervicalis
Later lobes displayed Plane of cleavage: b/w 2 capsules Vessels ligated and divided – STA right at the lower pole; ITA at some distance from lower pole During removal of gland Ligament of berry released – RLN injury Wedge shaped areas on post-medial surface is left behind- PT
Complications ELN injury – CT paralysis, hoarseness of voice, temporary until the other side takes over RLN injury – all intrinsic muscles except CT paralyzed, no recovery