science for life: Thyroid and Parathyroid Gland

Thyroid and Parathyroid Gland

Step 2: Structure of the Thyroid Gland (30 minutes)

· The thyroid controls how quickly the body burns energy, makes proteins, and controls how sensitive the body should be to other hormones.

· Iodine and tyrosine are used to form both T₃ and T₄. The thyroid also produces the hormone calcitonin, which plays a role in calcium homeostasis.

· The thyroid is controlled by the hypothalamus and pituitary. Hyperthyroidism (overactive thyroid) and hypothyroidism (underactive thyroid) are the most common problems of the thyroid gland.

Lymphatic drainage of Thyroid Gland Larynx and Trachea

Anatomy

· The thyroid gland is a butterfly-shaped organ and is composed of two cone-like lobes or wings: (right lobe) and (left lobe), and is also connected with the isthmus.

· The organ is situated on the anterior side of the neck, lying against and around the larynx and trachea, reaching posteriorly the oesophagus and carotid sheath.

· It starts cranially at the oblique line on the thyroid cartilage (just below the laryngeal prominence or Adam's apple) and extends inferiorly to the fifth or sixth tracheal ring.

· It is difficult to demarcate the gland's upper and lower border with vertebral levels because it moves position in relation to these during swallowing.

· The thyroid is supplied with arterial blood from the superior thyroid artery, a branch of the external carotid artery, and the inferior thyroid artery, a branch of the thyrocervical trunk, and sometimes by the thyroid artery, branching directly from the brachiocephalic trunk.

· The venous blood is drained via superior thyroid veins, draining in the internal jugular vein, and via inferior thyroid veins, left brachiocephalic vein.

· Lymphatic drainage passes frequently the lateral deep cervical lymph nodes and the pre- and parathracheal lymph nodes.

· The gland is supplied by sympathetic nerve input from the superior cervical ganglion and the cervicothoracic ganglion of the sympathetic trunk, and by parasympathetic nerve input from the superior laryngeal nerve and the recurrent laryngeal nerve.

Refer Handout 17.1: Position of Thyroid Gland

Thyroid Gland with its Major Vasculisation and Muscles

Physiology

· The primary function of the thyroid is production of the hormones thyroxine (T4), triiodothyronine (T3), and calcitonin.

· Up to 80% of the T4 is converted to T3 by peripheral organs such as the liver, kidney and spleen. T3 is about ten times more active than T4.

T3 and T4 Production and Action

· Thyroxine (T4) is synthesized by the follicular cells from free tyrosine and on the tyrosine residues of the protein called thyroglobulin (Tg).

· Iodine is captured with the "iodine trap" by the hydrogen peroxide generated by the enzyme thyroid peroxidase to T3.

· Thyroid hormone that is secreted from the gland is about 90% T4 and about 10% T3.

· Cells of the brain are a major target for the thyroid hormones T3 and T4. Thyroid hormones play a particularly crucial role in brain maturation during fetal development.

· In the blood, T4 and T3 are partially bound to thyroxine-binding globulin, transthyretin and albumin. Only a very small fraction of the circulating hormone is free (unbound) - T4 0.03% and T3 0.3%. Only the free fraction has hormonal activity.

T3 and T4 Regulation

· The production of thyroxine and triiodothyronine is regulated by thyroid-stimulating hormone (TSH), released by the anterior pituitary (that is in turn released as a result of TRH release by the hypothalamus). The thyroid and thyrotropes form a negative feedback loop. TSH production is suppressed when the T4 levels are high, and vice versa.

· The TSH production itself is modulated by thyrotropin-releasing hormone (TRH), which is produced by the hypothalamus and secreted at an increased rate in situations such as cold (in which an accelerated metabolism would generate more heat).

· TSH production is blunted by somatostatin (SRIH), rising levels of glucocorticoids and sex hormones (estrogen and testosterone), and excessively high blood iodide concentration.

Calcitonin

· An additional hormone produced by the thyroid contributes to the regulation of blood calcium levels.

· Parafollicular cells produce calcitonin in response to hypercalcemia. Calcitonin stimulates movement of calcium into bone, in opposition to the effects of parathyroid hormone (PTH). However, calcitonin seems far less essential than PTH, as calcium metabolism remains clinically normal after removal of the thyroid, but not the parathyroids.

Significance of Iodine

· In areas of the world where iodine (essential for the production of thyroxine, which contains four iodine atoms) is lacking in the diet, the thyroid gland can be considerably enlarged, resulting in the swollen necks of endemic goitre.

· Thyroxine is critical to the regulation of metabolism and growth throughout the animal kingdom.

· In humans, children born with thyroid hormone deficiency will have physical growth and development problems, and brain development can also be severely impaired, in the condition referred to as cretinism.

· Newborn children in many developed countries are now routinely tested for thyroid hormone deficiency as part of newborn screening by analysis of a drop of blood.

· Children with thyroid hormone deficiency are treated by supplementation with synthetic thyroxine, which enables them to grow and develop normally.

· The uptake mechanism with a large surplus of non-radioactive iodine, taken in the form of potassium iodide tablets. While biological researchers making compounds labelled with iodine isotopes do this, in the wider world such preventive measures are usually not stockpiled before an accident, nor are they distributed adequately afterward.

· The use of iodised salt is an efficient way to add iodine to the diet. It has eliminated endemic cretinism in most developed countries, and some governments have made the iodination of flour or salt mandatory. Potassium iodide and sodium iodide are the most active forms of supplemental iodine.

Step 3: Functions of The Thyroid Gland (30 minutes)

· Iodine is essential for the formation of the thyroid gland hormones, thyroxine (T4) and tri-iodothyronine (T3).

· The body's main sources of iodine are seafood, vegetables grown in iodine-rich soil and iodinated table salt in the diet. The thyroid gland selectively takes up iodine from the blood, a process called iodine trapping.

· The thyroid hormones are synthesised as large preecursor molecules called thyroglobulin, the major constituent of colloid. The release of T3 and T4 into the blood is regulated by thyroid stimulating hormone (TSH) from the anterior pituitary.

· Secretion of TSH is stimulated by thyroid releasing hormone (TRH) from the hypothalamus and secretion of TRH is stimulated by exercise, stress, malnutrition, low plasma glucose and sleep.

· Secretion of T3 and T4 begins about the third month of fetal life and is increased at puberty and in women during the reproductive years, especially during pregnancy. Otherwise, it remains fairly constant throughout life.

· Thyroid hormones enter the target cells and regulate the expression of genes in the nucleus, i.e. they increase or decrease the synthesis of some proteins including enzymes. They combine with specific receptor sites and enhance the effects of other hormones, e.g. adrenaline (epinephrine) and noradrenaline (norepinephrine).

T3 and T4 Affect Most Cells of the Body By

· Increasing the basal metabolic rate and heat production

· Regulating metabolism of carbohydrates, proteins and fats.

· T3 and T4 are essential for normal growth and development, especially of the skeleton and nervous system.

· Most other organs and systems are also influenced by thyroid hormones. Physiological effects of T3 and T4 on the heart, skeletal muscles, skin, digestive and reproductive systems are more evident when there is underactivity or overactivity of the thyroid gland.

Calcitonin

· This hormone is secreted by the parafollicular or C-cells in the thyroid gland (Fig. 9.. It acts on bone and the kidneys to reduce the blood calcium (Ca2+) level when it is raised.

· It reduces the reabsorption of calcium from bones and inhibits reabsorption of calcium by the renal tubules. Its effect is opposite to that of parathyroid hormone, the hormone secreted by the parathyroid glands.

· Release of calcitonin is stimulated by an increase in the blood calcium level.

· This hormone is important during childhood when bones undergo considerable changes in size and shape.

: Thyroid and Parathyroid Glands and their Roles in the Control of Calcium

Step 4: Structure of the Parathyroid Gland (10 minutes)

· Two parathyroid glands lie against the posterior surface of each lobe and are sometimes embedded in thyroid tissue.

· The recurrent laryngeal nerve passes upwards close to the lobes of the gland and on the right side it lies near the inferior thyroid artery.

· The gland is composed of cuboidal epithelium that forms spherical follicles. These secrete and store colloid, a thick sticky protein material. Between the follicles there are other cells found singly or in small groups, para follicular cells, also called C-cells, which secrete the hormone calcitonin

Step 5: Functions of the Parathyroid Gland (25 minutes)

· Regulation of serum calcium.

· Parathyroid hormone regulates serum calcium levels through its effects on the following tissues,

Region	Effect
bone	It enhances the release of calcium from the large reservoir contained in the bones. Bone resorption is the normal destruction of bone by osteoclasts, which are indirectly stimulated by PTH. Stimulation is indirect since osteoclasts do not have a receptor for PTH; rather, PTH binds to osteoblasts, the cells responsible for creating bone.
kidney	It enhances active reabsorption of calcium and magnesium from distal tubules and the thick ascending limb. As bone is degraded both calcium and phosphate are released. It also greatly increases the excretion of phosphate, with a net loss in plasma phosphate concentration. By increasing the calcium: phosphate ratio more calcium is therefore free in the circulation.
intestine via kidney	It enhances the absorption of calcium in the intestine by increasing the production of activated vitamin D. Vitamin D activation occurs in the kidney. PTH up-regulates 25-hydroxyvitamin D3 1-alpha-hydroxylase, the enzyme responsible for 1-alpha hydroxylation of 25-hydroxy vitamin D, converting vitamin D to its active form (1,25-dihydroxy vitamin D). This activated form of vitamin D increases the absorption of calcium (as Ca²⁺ ions) by the intestine via calbindin.

Regulation of Serum Phosphate

· PTH reduces the reabsorption of phosphate from the proximal tubule of the kidney, which means more phosphate is excreted through the urine.

· However, PTH enhances the uptake of phosphate from the intestine and bones into the blood. In the bone, slightly more calcium than phosphate is released from the breakdown of bone.

· In the intestines, which is mediated by an increase in activated vitamin D, the absorption of phosphate is not as dependent on vitamin D as is that of calcium. The end result is a small net drop in the serum concentration of phosphate.

Vitamin D Synthesis

· PTH increases the activity of 1-α-hydroxylase enzyme, which converts 25-hydroxycholecalciferol to 1, 25-dihydroxycholecalciferol, the active form of vitamin D.

Regulation of PTH Secretion

· Secretion of parathyroid hormone is chiefly controlled by serum [Ca²⁺] through negative feedback, which is achieved by the activation of calcium-sensing receptors located on parathyroid cells.

Stimulators

· Decreased serum [Ca²⁺].

· Mild decreases in serum [Mg²⁺].

· An increase in serum phosphate (Since increased phosphate will complex with serum calcium to form calcium phosphate, this causes the Ca sensitive receptors (CaSr) to think that serum Ca has decreased, as CaSR do not sense Calcium phosphate, thereby triggering an increase in PTH)

Inhibitors

· Increased serum [Ca²⁺].

· Severe decreases in serum [Mg²⁺], which also produces symptoms of hypoparathyroidism (such as hypocalcemia).

Clinical Significance

· A high level of PTH in the blood is known as hyperparathyroidism.

o If the cause is in the parathyroid gland it is called primary hyperparathyroidism. The causes are parathyroid adenoma, parathyroid hyperplasia and parathyroid cancer.

o If the cause is outside the gland, it is known as secondary hyperparathyroidism. This can occur in chronic renal failure. In secondary hyperparathyroidism, serum Calcium levels are decreased, which causes the hypersecretion of PTH from the parathyroid glands. PTH acts on the proximal tubules in the kidney to decrease reabsorption of Phosphate (increasing its excretion in urine, decreasing its serum concentration). Note: however, in chronic renal failure, because the kidneys are failing they are unable to excrete phosphate in the urine, so in this case of secondary hyperparathyroidism, serum calcium will be decreased, but serum phosphate will be increased.

· A low level of PTH in the blood is known as hypoparathyroidism. Causes include surgical misadventure (eg inadvertent removal during routine thyroid surgery), autoimmune disorder, and inborn errors of metabolism.

Measurement

· 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.

Step 6: Key Points (5 minutes)

· The thyroid is one of the largest endocrine glands in the body. This gland is found in the neck inferior to (below) the thyroid cartilage (also known as the Adam's apple in men) and at approximately the same level as the cricoid cartilage. The thyroid controls how quickly the body burns energy, makes proteins, and controls how sensitive the body should be to other hormones.

· The thyroid participates in these processes by producing thyroid hormones, principally thyroxine (T₄) and triiodothyronine (T₃). These hormones regulate the rate of metabolism and affect the growth and rate of function of many other systems in the body. Iodine and tyrosine are used to form both T₃ and T₄. The thyroid also produces the hormone calcitonin, which plays a role in calcium homeostasis.

· T3 and T4 affect most cells of the body by,

· Increasing the basal metabolic rate and heat production

· Regulating metabolism of carbohydrates, proteins and fats.

· T3 and T4 are essential for normal growth and development, especially of the skeleton and nervous system. Most other organs and systems are also influenced by thyroid hormones. Physiological effects of T3 and T4 on the heart, skeletal muscles, skin, digestive and reproductive systems are more evident when there is underactivity or overactivity of the thyroid gland.

· Two parathyroid glands lie against the posterior surface of each lobe and are sometimes embedded in thyroid tissue. The recurrent laryngeal nerve passes upwards close to the lobes of the gland and on the right side it lies near the inferior thyroid artery.

· Parathyroid hormone regulates serum calcium levels.

1: Position of Thyroid gland

Step no 1. Presentation of Session Title and Learning Objectives (5 minutes)

READ or ASK students to read the learning objectives and clarify.

ASK students if they have any questions before continuing.

Step no 2. Sensory organs of the human body.(10minutes)

Activity: Brainstorming (5 minutes)

Ask students following question:-

· Mention one sensory organ of the human body:-

Record the responses and write them on a flip chart.

Conclude: by listing five sensory organs as follows:-

· The following are the five sensory organs of the human body.

o Ear.

o Eye.

o Nose.

o Tongue.

o Skin.

Step no 3. Organization of each sensory organ.( 60minutes)

THE EARS:-

The ear is organized as follows:-

· Ears are paired sensory organs comprising,

o The auditory system, involved in the detection of sound, and

o The vestibular system, involved with maintaining body balance/ equilibrium.

· The ear divides anatomically and functionally into three regions

The external ear,
The middle ear, and
The inner ear.

· All three regions are involved in hearing. Only the inner ear functions in the vestibular system.

· The external ear (or pinna, the part you can see) serves to protect the tympanic membrane (eardrum), as well to collect and direct sound waves through the ear canal to the eardrum. About 1¼ inches long, the canal contains modified sweat glands that secrete cerumen, or earwax. Too much cerumen can block sound transmission.

· The middle ear, separated from the external ear by the eardrum, is an air-filled cavity (tympanic cavity) carved out of the temporal bone. It connects to the throat/nasopharynx via the Eustachian tube.

· This ear-throat connection makes the ear susceptible to infection (otitis media). The eustachian tube functions to equalize air pressure on both sides of the eardrum. Normally the walls of the tube are collapsed.

· Swallowing and chewing actions open the tube to allow air in or out, as needed for equalization. Equalizing air pressure ensures that the eardrum vibrates maximally when struck by sound waves.

· Adjoining the eardrum are three linked, movable bones called ossicles, which convert the sound waves striking the eardrum into mechanical vibrations.

· The smallest bones in the human body, the ossicles are named for their shape. The hammer (malleus) joins the inside of the eardrum. The anvil (incus), the middle bone, connects to the hammer and to the stirrup (stapes). The base of the stirrup, the footplate, fills the oval window which leads to the inner ear.

· The inner ear consists of a maze of fluid-filled tubes, running through the temporal bone of the skull. The bony tubes, the bony labyrinth, are filled with a fluid called perilymph. Within this bony labyrinth is a second series of delicate cellular tubes, called the membranous labyrinth, filled with the fluid called endolymph.

· This membranous labyrinth contains the actual hearing cells, the hair cells of the organ of Corti. There are three major sections of the bony labyrinth,

o The front portion is the snail-shaped cochlea, which functions in hearing.

o The rear part, the semicircular canals, helps maintain balance.

o Interconnecting the cochlea and the semicircular canals is the vestibule, containing the sense organs responsible for balance, the utricle and saccule.

· The inner ear has two membrane-covered outlets into the air filled middle ear, the oval window and the round window.

· The oval window sits immediately behind the stapes, the third middle ear bone, and begins vibrating when "struck" by the stapes. This sets the fluid of the inner ear sloshing back and forth.

· The round window serves as a pressure valve, bulging outward as fluid pressure rises in the inner ear. Nerve impulses generated in the inner ear travel along the vestibulocochlear nerve (cranial nerve VIII), which leads to the brain.

· This is actually two nerves, somewhat joined together, the cochlear nerve for hearing and the vestibular nerve for equilibrium.

THE EYE

The eye is organized into the following major parts :( Orbital cavity, eye balls)

· The eye is an organ of the sense of sight situated in the orbital cavity

· It is almost spherical in shape and is about 2,5 cm in diameter

· The space between the eye and the orbital cavity is occupied by adipose tissue. The bony walls of the orbit and the fat help to protect the eye from injury.

· Human being has two eyes. They function as a pair.

The orbital cavity.

o Seven bones contribute to the framework of each orbital cavity. They are the maxilla, zygomatic, frontal, ethmoid, lacrimal, sphenoid, and palatine bones.

o Together they give the bony orbital cavity the shape of a pyramid, with its wide base opening anteriorly onto the face, and its apex extending in a posteromedial direction.

o There are two groups of muscles within the orbit,

- Extrinsic muscles of eyeball (extra-ocular muscles)

- Intrinsic muscles within the eyeball .

o The arteries of the orbit are mainly from the ophthalmic artery, a branch of the internal carotid artery and the infraorbital artery, from the external carotid artery, also contributes to the supply of this region.

o There are two venous channels in the orbit, the superior and inferior ophthalmic veins

o Numerous nerves pass into the orbit and innervate structures within its bony walls. They include

- The optic nerve [II]

- The oculomotor nerve [III]

- The trochlear nerve [IV]

- The abducent nerve [VI]

- Autonomic nerves.

- Other nerves such as the ophthalmic nerve [V₁] innervate orbital structures and then travel out of the orbit to innervate other regions

o The eyelids and lacrimal fluid, secreted by the lacrimal glands, protect the cornea and eyeball from injury and irritation.

The Eye ball.

o The globe shaped eyeball occupies the anterior part of the orbit. Its rounded shape is disrupted anteriorly, where it bulges outward. This outward projection represents about one sixth of the total area of the eyeball and is the transparent corn ea

Posterior to the cornea and in order from front to back are the anterior chamber, the iris and pupil, the posterior chamber, the lens, the postrenal (vitreous) chamber, and the retina

o The anterior chamber and posterior chambers.

- The anterior chamber is the area directly posterior to the cornea and anterior to the colored part of the eye (iris). The central opening in the iris is the pupil. Posterior to the iris and anterior to the lens is the smaller posterior chamber.

- The anterior and posterior chambers are continuous with each other through the pupillary opening. They are filled with a fluid (aqueous humor), which is secreted into the posterior chamber, flows into the anterior chamber through the pupil, and is absorbed into the scleral venous sinus

- The aqueous humor supplies nutrients to the avascular cornea and lens and maintains the intraocular pressure. If the normal cycle of its production and absorption is disturbed so that the amount of fluid increases, intraocular pressure will increase. This condition (glaucoma) can lead to a variety of visual problems.

- Lens and vitreous humor. The lens separates the anterior one-fifth of the eyeball from the posterior four-fifths. It is a transparent, biconvex elastic disc attached circumferentially to muscles associated with the outer wall of the eyeball. This lateral attachment provides the lens with the ability to change its refractive ability to maintain visual acuity. The clinical term for opacity of the lens is a cataract

The posterior four-fifths of the eyeball, from the lens to the retina, is occupied by the postrenal (vitreous) chamber. This segment is filled

science for life

Sunday, August 10, 2014

Thyroid and Parathyroid Gland