Thyroid gland biochemistry mbbs 2nd year .pptx

this is presentation about the thyroid gland specifically its biochemistry and physiology that are included in mbbs 2nd year course

1. Thyroid Gland

2. Location The thyroid gland is located in the anterior neck, just below the larynx Right and Left Lobes: These are the main parts of the gland Isthmus: This thin strip of tissue connects the two lobes Weight 15-20 gram

3. Histology of Thyroid gland Thyroid Follicles: Spherical structures forming the main functional units of the thyroid gland. Lined by a single layer of follicular cells and filled with colloid, which contains thyroglobulin, essential for thyroid hormone production. Follicular Cells (Thyrocytes): Cuboidal to columnar epithelial cells lining the follicles. Responsible for synthesizing and secreting the thyroid hormones T3 and T4. Cell shape varies based on activity level; more columnar when active, squamous when inactive. Parafollicular Cells (C Cells): Larger, pale-staining cells located between the follicles. Produce calcitonin, a hormone involved in regulating blood calcium levels. Not involved in thyroid hormone production. 3 Follicular Cells  produce  thyroxine (T 4 )  and  triiodothyronine (T 3 ) . Whilst T 4  is usually produced in greater quantities than T 3 , T 3  is up to 4 times more potent than T 4 . Additionally, about a third of T 4  is converted to T 3  within peripheral tissues, especially within the lungs and the liver. Parafollicular Cells  produce calcitonin (CT)

4. Synthesis Hormones of thyroid gland 4 Iodine Uptake: The thyroid gland actively absorbs iodine from the bloodstream through a process called iodide trapping mediated by the sodium-iodide symporter (NIS) in the follicular cells. Thyroglobulin Production: Follicular cells synthesize thyroglobulin, a large glycoprotein, which is secreted into the follicular lumen (colloid). Iodination and Formation of T3/T4: Iodine is oxidized by the enzyme thyroid peroxidase (TPO ) and binds to tyrosine residues on thyroglobulin, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT).Coupling of these iodinated tyrosines results in the formation of triiodothyronine (T3) and thyroxine (T4). Release into Circulation: Thyroglobulin with bound T3 and T4 is endocytosed back into follicular cells, where it is cleaved by proteases, releasing T3 and T4 into the bloodstream. Regulation: The synthesis and release of thyroid hormones are regulated by thyroid-stimulating hormone (TSH) from the pituitary gland, which is controlled by thyrotropin-releasing hormone (TRH) from the hypothalamus in a negative feedback loop involving circulating levels of T3 and T4.

5. Hormones of thyroid gland 5 Thyroxine (T4) Composed of four iodine atoms attached to a tyrosine amino acid. Synthesized in the thyroid follicular cells, T4 is the major hormone produced by the thyroid gland and is converted into the more active form, T3, in peripheral tissues. T4 is present in higher concentrations in the blood compared to T3, with about 80-90% of circulating thyroid hormones being T4. Triiodothyronine (T3) Composed of three iodine atoms attached to a tyrosine amino acid. It is the more active form of thyroid hormone and is produced both by the thyroid gland and through conversion of T4 in peripheral tissues. T3 constitutes a smaller fraction of circulating thyroid hormones (approximately 10-20% of the total thyroid hormones) but is more potent in its biological effects compared to T4. Calcitonin A peptide hormone produced by the parafollicular cells (C cells) of the thyroid gland. It helps regulate calcium levels in the blood by inhibiting osteoclast activity in the bones and reducing calcium release.

6. Mechanism of action Thyroid hormones (T3 and T4) enter target cells, where T4 is often converted to the more active T3 by the enzyme deiodinase. T3 then binds to nuclear thyroid hormone receptors (TRα and TRβ). These receptors, often in complex with retinoid X receptor (RXR), bind to thyroid hormone response elements (TREs) in the DNA, regulating the transcription of genes involved in metabolism, growth, and development. This regulation increases metabolic rate, oxygen consumption, and influences cardiovascular function, among other effects. Thyroid hormone levels are tightly controlled by feedback mechanisms involving the hypothalamus and pituitary gland. 6

7. Thyroid hormones increase cellular metabolic activity and basal metabolic rate: They increase the number and activity of the mitochondria leading to more production of ATP they can increase transport of ions through the cell membrane due to increased Na K atapase activity Growth hormone has role in growth of the body also has role in growth and development of brain in fetal life Stimulation of carbohydrate metabolism(increased glucose uptake glycolysis and gluconeogenis and increased absorption from the gut) Thyroid hormones increase the metabolic rate, leading to enhanced breakdown of plasma lipids and reduced liver fat accumulation Increased requirement for vitamin due to increased BMR Decreased body weight Increased blood flow and cardiac output Increased heart rate 7 Functions of thyroid Hormone

8. Increased respiration Increased GIT motility In low concentration enhances muscle activity but in high they become weakened due to increased protein metabolism In excess amount can cause characteristic muscle tremors Thyroid hormones influence sleep by regulating metabolic processes and energy levels; abnormal thyroid function can lead to sleep disturbances, with hyperthyroidism often causing insomnia and hypothyroidism potentially leading to excessive daytime sleepiness. Thyroid hormones play a critical role in sexual function by regulating libido, menstrual cycles, and fertility. Hypothyroidism can lead to reduced libido, irregular menstrual cycles, and difficulty conceiving, while hyperthyroidism may cause menstrual irregularities and decreased sexual desire. 8 Functions of thyroid Hormone

9. Regulation Regulation is Through Hypothalamic-Pituitary-Thyroid Axis

10. Regulation Hypothalamic-Pituitary-Thyroid Axis: The hypothalamus secretes thyrotropin-releasing hormone (TRH), which stimulates the anterior pituitary gland. The anterior pituitary releases thyroid-stimulating hormone (TSH) in response to TRH. TSH Secretion: TSH stimulates the thyroid gland to produce and release thyroid hormones (T3 and T4). Thyroid Hormone Feedback: T3 and T4 exert negative feedback on the pituitary gland and hypothalamus to regulate TRH and TSH levels. High levels of T3 and T4 decrease TRH and TSH secretion, while low levels increase their secretion. Peripheral Conversion: In peripheral tissues, T4 (thyroxine) is converted to the more active T3 (triiodothyronine) by deiodinase enzymes, modulating local thyroid hormone activity.

11. Diseases of thyroid Graves' disease is an autoimmune disorder where the immune system produces antibodies that stimulate the thyroid gland, leading to hyperthyroidism. This results in excessive production of thyroid hormones, causing symptoms such as weight loss, rapid heartbeat, tremors, and bulging eyes (exophthalmos). The condition often requires treatment to manage hormone levels and reduce autoimmune activity.

12. Diseases of thyroid Goiter is an enlargement of the thyroid gland commonly caused by iodine deficiency, which impairs thyroid hormone production and prompts the gland to increase in size to compensate. It can also result from autoimmune diseases such as Hashimoto's thyroiditis. Signs and symptoms include a visible swelling in the neck, difficulty swallowing or breathing if the goiter becomes large, and potential changes in thyroid function, which might lead to symptoms of hyperthyroidism (e.g., weight loss, rapid heartbeat) or hypothyroidism (e.g., fatigue, weight gain), depending on the underlying cause.

13. Diseases of thyroid Cretinism is a condition resulting from severe congenital hypothyroidism, often due to inadequate thyroid hormone production during fetal development. This can be caused by thyroid gland abnormalities or iodine deficiency. The condition leads to stunted physical growth, developmental delays, and intellectual disabilities. Infants may present with a low, hoarse cry, poor feeding, and sluggishness. Early diagnosis and treatment with thyroid hormone replacement therapy are crucial to prevent or minimize developmental impairments . Myxedema is a severe form of hypothyroidism characterized by significant, widespread swelling of the skin and tissues due to the accumulation of mucopolysaccharides (glycosaminoglycans) in the extracellular matrix. This condition typically results from prolonged, untreated hypothyroidism. Symptoms include puffiness of the face, swelling of the extremities, cold intolerance, and a hoarse voice. In severe cases, it can lead to myxedema coma, a life-threatening emergency requiring immediate medical intervention with thyroid hormone replacement therapy.

14. Thank you! BRITA TAMM 502-555-0152 [email protected] www.firstupconsultants.com