ENDOCRINE SYSTEM| CHEMICAL CONTROL AND COORDINATION| CHSE +2 1st yr ZOOLOGY|B.sc| OUAT|OJEE| B.ed

Описание: 1. Location and Structure

Location: Anterior neck, just below the larynx, wrapping around the trachea.

Shape: Butterfly-shaped with two lateral lobes connected by a thin isthmus.

Weight: ~15–25 g in adults.

Blood Supply:

Arteries: Superior thyroid artery (from external carotid) and inferior thyroid artery (from thyrocervical trunk).

Veins: Superior, middle, and inferior thyroid veins draining into internal jugular and brachiocephalic veins.


Innervation: Sympathetic fibers from the cervical ganglia.



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2. Histology

Follicles: Spherical structures lined with cuboidal epithelial cells (follicular cells).

Colloid: Central lumen filled with thyroglobulin, the precursor of thyroid hormones.

C cells (Parafollicular cells): Scattered between follicles; produce calcitonin.



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3. Hormones Secreted

A. Thyroid Hormones

1. Thyroxine (T4)

Contains 4 iodine atoms.

Accounts for ~90% of thyroid secretion.

Relatively inactive; converted to T3 in target tissues.



2. Triiodothyronine (T3)

Contains 3 iodine atoms.

More active than T4 (~3–5 times).

Formed by peripheral conversion of T4 via 5’-deiodinase.




Functions of T3 & T4:

Regulate basal metabolic rate (BMR).

Promote growth and development, especially of the nervous system.

Increase oxygen consumption and heat production.

Influence carbohydrate, lipid, and protein metabolism.

Stimulate heart rate and cardiac output.


B. Calcitonin

Produced by C cells.

Lowers blood calcium by:

Inhibiting osteoclast activity in bones.

Reducing calcium reabsorption in kidneys.


Plays a minor role in calcium homeostasis in adults.



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4. Thyroid Hormone Synthesis

1. Iodide uptake: By follicular cells via sodium-iodide symporter.


2. Oxidation: Iodide → iodine via thyroid peroxidase (TPO).


3. Iodination of thyroglobulin: Forms MIT (monoiodotyrosine) and DIT (diiodotyrosine).


4. Coupling: MIT + DIT = T3; DIT + DIT = T4.


5. Storage: Hormones stored in colloid as thyroglobulin.


6. Release: Endocytosis of colloid → lysosomal proteolysis → secretion of T3/T4.




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5. Regulation

Controlled by hypothalamic-pituitary-thyroid axis:

TRH (thyrotropin-releasing hormone) from hypothalamus → stimulates TSH from anterior pituitary → stimulates thyroid follicular cells to release T3 & T4.


Feedback: High T3/T4 inhibit TRH & TSH (negative feedback).



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6. Disorders

Hypothyroidism

Causes: Iodine deficiency, Hashimoto’s thyroiditis, congenital (cretinism).

Symptoms: Fatigue, weight gain, cold intolerance, slow heart rate, goiter (sometimes).

Special types:

Cretinism: Congenital; dwarfism & mental retardation.

Myxedema: Adult hypothyroidism; edema, thick skin.



Hyperthyroidism

Causes: Graves’ disease (autoimmune), toxic adenoma.

Symptoms: Weight loss, heat intolerance, rapid heart rate, goiter, exophthalmos.


Other Disorders

Goiter: Enlargement of thyroid (iodine deficiency common cause).

Thyroid cancer: Papillary, follicular, medullary, anaplastic types.



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7. Clinical Significance
Thyroid function tests (TFTs): Measure T3, T4, TSH levels.

Iodine requirement: ~150 µg/day.

Radioactive iodine (I-131): Used in hyperthyroidism treatment.



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