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

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