Anatomy and physiology of thyroid gland
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Transcript of Anatomy and physiology of thyroid gland
ANATOMY AND PHYSIOLOGY OF THYROID GLAND
DR MANOHAR, RESIDENT INHS ASVINI
Embryology Macroscopic anatomy Microscopic anatomy Physiology
Median anlage-midline thickening of ventral surface between 1&2 Branchial arches
Diverticulum at 16 or 17 day-Foramen caecum
Bilobed structure
Thyroglossal duct cyst-persistence of stalk
most common between hyoid bone & isthmus
Embryology
Thyroglossal duct cyst
Lateral anlagen-ultimobranchial bodies from 4 or 5 pouches
fuse with median by 6th week
Lateral thyroid lobe-tendency for neoplasia
C-cells-numerous at point of fusion
Medullary carcinoma
Inferior parathyroid –dorsal wing of 3rd pharyngeal pouch Superior parathyroid- dorsal wing of 4th
pharyngeal pouch
15 to 25 gm
rarely one lobe fails to develop-left
Apex narrow upto oblique line of thyroid cart
Rounded lower pole upto 4th or 5th tra. Ring lateral to trachea and oesophagus, medial to to carotid sheath
Pyramidal lobe in 50%
Macroscopic Anatomy
Tubercle of Zuckerkandl in 60% point of fusion ultimobranchial bodies and median anlage
Surgical Imp: 1. recurrent laryngeal N runs medially 2. sup parathyroid gland attached cranially 3. when enlarged can be left behind
Thyroid gland Fascia Invested in pretracheal fascia Suspensory ligament of berry-small blood vessels divided with care as recurrent Laryngeal N may
lie medially, laterally or in ligament Laterally fascia blends with carotid sheath Inferiorly blends with adventitia of arch of aorta
Thyroid gland fascia
Arterial Supply Superior thyroid artery-external carotid A (a) Anterior and (b) posterior branch at superior pole
Posterior branch descends on posterior surface
In 45% joins with ascending branch of inferior thyroid artery
Inferior thyroid artery thyrocervical trunk from subclavian artery in 15% divides in ascending and descending branch
Inferior Laryngeal artery ascends with recurrent laryngeal N to enter larynx deep to inferior constrictor ms accompanied with plexus of veins in region of
ligament of berry Thyroidea ima art in 12% from brachiocephal artery
Superior thyroid vein-internal jagular vein
Middle thyroid vein blood from inferior and anterolateral part & larynx and trachea cross common carotid and drain in IJV
Inferior thyroid vein plexus on trachea and drain into (R) & (L) inferior veins-respective brachiocephalic vein In few paired inferior form common trunk- SVC
Venous drainage
Autonomic and Harmonal control Sympathetic from superior cervical and stellate ganglion Parasympathetic from nodose and local vagal ganglia
Left recurrent laryngeal N passes behind inferior thyroid artery & then post to ligament of berry before passing between
fibres of cricopharyngeal part of inferior constrictor, it lies behind capsule of cricothyroid j
(R) recurrent-loops 1st part of subclavian artery
Nerves
Non recurrent lar N in 0.3 to 0.8%(R) & 0.004%(L)
lateral aspect to level III, IV and V
more medial into level VI in turn in upper anterior mediastinum level VII
level I& II rarely involved
Lymphatic drainage
follicular cells around central pool of colloid follicles spherical 0.02 & 0.9mm in
diameter thyroid lobule consist of 20-40 follicles follicular cells long rough endoplasmic
reticulum and large golgi apparatus, prominent lysosomal bodies
Inactive gland-cells flattened and abundant colloid
On stimulation with TSH-columnar shape
Microscopic Anatomy
Each follicle invested in loose connective tissue consists of plexus of capillaries& lymphatics
Interfollicular connective tissue consists of fibroblasts, unmyelinated nerve fibres with
schwann cells,fat cells, plasma cells,macrophage ,lymphocytes
Calcitonin producing C-cells singly or in small clumps adjacent to stromal aspect of follicular cells
Physiology of thyroid gland Regulation of thyroid gland metabolism
The thyroid gland synthesizes and secretes three hormones:
• Thyroxine (T4). • Tri-iodothyronine (T3). • Calcitonin
• Dietary iodine is absorbed in the GI tract• 90% excreted in kidney
• The transport of iodide into follicular cells is dependent upon a Na+/I- cotransport system.
• Iodide taken up by the thyroid gland is oxidized by peroxide in the lumen of the follicle
• Oxidized iodine can then be used in production of thyroid hormones.
Iodine metabolism and Iodine trapping
Iodine trapping
Daily iodine requirement-150 microgram/d
Less than 50 microgram/day-goitre
Selenium
Iodine excess- inhibits iodide oxidation organification and thyroglobulin proteolysis
Pituitary produces TSH, which binds to follicle cell receptors.
The follicle cells of the thyroid produce thyroglobulin.
Thyroglobulin incorporated in apical vesicles At apical membrane thyroid peroxidase use
H2O2 and iodide to oxidize and organify thyrogloulin protein into MIT and DIT as well as some T4&T3.
Iodification of thyroglobulin
Secretion of Thyroid harmone
• They are found in the circulation associated with binding proteins:- Thyroid Hormone-Binding Globulin (~70% of hormone)- Pre-albumin (transthyretin), (~15%)- Albumin (~15%)
• Less than 1% of thyroid hormone is found free in the circulation.
• Only free and albumin-bound thyroid hormone is biologically available to tissues.
Thyroid harmone transport
Enter cell by diffusion, active transport, reach endoplasmic reticulum where T4- T3 Intracellular T3 acts on specific nuclear receptors (members of c-erbA superfamily)
Thyroid hormones are essential for normal growth of tissues, including the nervous system.
Lack of thyroid hormone during development results in short stature and mental deficits (cretinism).
Thyroid hormone stimulates basal metabolic rate
Actions of thyroid harmone
Required for GH and prolactin production and secretion
Increases intestinal glucose reabsorption
Increases mitochondrial oxidative phosphorylation (ATP production)
Increases activity of adrenal medulla
• Effects on protein synthesis and degradation:-increased protein synthesis at low thyroid hormone levels (low metabolic rate)-increased protein degradation at high thyroid hormone levels (high metabolic rate)
• Effects on carbohydrates:-low doses of thyroid hormone increase
glycogen synthesis (low metabolic rate)- high doses increase glycogen breakdown (high metabolic rate)
Excess thyroid harmone secretion:-
1. Increased O2 consumption 2. Weight loss (protein, fat cataolism) 3. skeletal muscle catabolism-hypercalcemia osteoporosis-mobilization of bone protein
Increase heart rate
Increase force of cardiac contractions
Increase stroke volume
Increase Cardiac output
Effects of thyroid harmone on CVS
Increase resting respiratory rate
Increase minute ventilation
Increase ventilatory response to hypercapnia and hypoxia
On respiratory system
Increase blood flow
Increase glomerular filtration rate
On renal system
Increase growth and maturation of bone tooth development and eruption growth and maturation of epidermis,hair follicles
and nails Increase rate and force of skeletal muscle
contraction Inhibits synthesis and increases degradation of
mucopolysaccharides in subcutaneous tissue
On growth and tissue development
Critical for normal CNS neuronal development
Enhances wakefulness and alertness
Enhances memory and learning capacity
Required for normal emotional tone
Increase speed and amplitude of peripheral nerve reflexes
On CNS
Required for normal follicular development and ovulation in the female
Required for the normal maintenance of pregnancy
Required for normal spermatogenesis in the male
On Reproductive System
Early onset: delayed/incomplete physical and mental development
Later onset (youth): Impaired physical growth Adult onset (myxedema) : gradual changes occur.
Tiredness, lethargy, decreased metabolic rate, slowing of mental function and motor activity, cold intolerance, weight gain, goiter, hair loss, dry skin. Eventually may result in coma.
Hypothyroidism
Calcitonin in response to elevated levels of calcium or gastrin. It lowers serum calcium and phosphate by inhibiting osteoclastic resorption of bone & enhance excretion by kidneys.
Calcitonin
Thank You