Location:
Present in the neck region just below the larynx.
Structure:
Weight is 30 gms.
Butterfly shaped.
Two lobes, the right and left lobe, are joined by the middle part called “Isthmus”.
Each of the thyroid lobes are embedded with parathyroid glands, primarily on their posterior surfaces
Thyroid gland is made up of a large no. of follicles called “Thyroid Follicles”.
Follicles contain a lumen containing a ground substance called “Colloid” surrounded by cuboidal epithelial cells called “Follicular cells”.
Some cells that can not reach the lumen are called “Parafollicular cells / C Cells (Clear Cells)”.
Colloid is a sticky liquid that is the center for production of thyroid hormones T3 and T4 from an essential item “Iodine”.
C cells secrete a hormone called “Calcitonin” which plays a role in calcium homeostasis.
Synthesis Storage & Release of Thyroid Hormones:
Steps involved in thyroid hormone synthesis,
Thyroglobulin synthesis.
Iodide trapping.
Oxidation of iodide to iodine.
Iodination of thyroglobulin
Coupling of two iodinated tyrosine molecules to form T3 / T4.
Secretion.
Under the influence of TSH, follicular cells start trapping iodide ions (I-) from blood. (Concn. Of “I” is much higher in cells than in blood).
In the lumen of follicular cells the two iodide ions get oxidized to form a molecular iodine molecule “I2” under the influence of an enzyme called “Thyroperoxidase”.
The formed I2 is then released in the colloid.
The follicular cells also release a protein called “Thyroglobulin” in the colloid.
The colloid Thyroid Peroxidase enzyme links the “Tyrosine” molecule with iodine.
Two intermediate are formed
Tyrosine is attached with one iodine.
Tyrosine is attached with two iodine molecules.
These intermediates fuse with each other to form,
Triiodothyronine (T3)
Tetraiodothyronine / Thyroxine (T4).
The hormones in colloid remain bound with Thyroglobulin.
Under the influence of TSH the colloid is taken inside the follicular cells by endocytosis.
Inside the follicular cell the lysosomal enzymes break down the thyroglobulin and free the hormones T3 and T4.
T3 and T4 are lipid soluble diffuses from the cell membrane into the bloodstream.
In blood only 1% of thyroid hormones remain unbound.
99% of thyroid hormones are bound to special proteins called “thyroxine-binding globulins (TBGs)”, albumins and other plasma proteins.
As plasma concentration of unbound thyroid hormones falls, the bounded hormones get released to maintain the ratio.
Regulation of thyroid hormone synthesis:
In response to decreased blood levels of T3 & T4, Hypothalamus secretes a hormone called “Thyrotropin releasing hormone (TRH)” to the pituitary gland.
Pituitary gland in response to TRH releases “Thyroid Stimulating Hormone (TSH) / Thyrotropin”.
Thyroid glands under influence of TSH secrete T3 and T4 in blood.
Increased levels of T3 & T4 in blood inhibits secretion of TRH & TSH.
Functions of Thyroid Hormones:
Thyroid hormones attach their receptors present on mitochondria and increase Basal Metabolic Rate (BMR).
Also stimulate formation of ATPs in the cell.
Stimulates carbohydrate, protein and fat metabolism.
Essential for normal brain development.
Essential for physical as well as mental growth.
Along with sex hormones they are involved in maintaining fertility.
Disorders of Thyroid Gland:
Cretinism:
Also called “Neonatal Hypothyroidism”.
It is caused due to low levels of thyroid hormones during birth.
It causes severe mental retardness and bone deformities, hearing and speech problems.
Myxedema:
Caused due to decreased thyroid activity (hypothyroidism).
Characterized by facial edema (swelling), lethargy, low body temp., pale skin, muscular weakness, lowered heart rate.
Grave’s Disease:
Autoimmune disease.
Body produces antibodies that mimic the action of TSH.
It results in hyperactivity of the Thyroid gland.
It is characterized by an increased metabolic rate, excessive body heat and sweating, diarrhea, weight loss, tremors, and increased heart rate.
The person's eyes bulge out called “Exophthalmos”
Goiter:
It results due to deficiency of iodine.
It is characterized by hypothyroidism.
Due to continuous stimulation by TSH the size of thyroid gland increases.
Calcitonin:
It is secreted by “C” cells (Parafollicular cells).
Released in response to increased blood calcium levels.
It decreases blood calcium levels by following mechanisms,
Inhibiting the activity of osteoclasts, bone cells that release calcium into the circulation by degrading bone matrix
Increasing osteoblastic activity
Decreasing calcium absorption in the intestines
Increasing calcium loss in the urine
Sometimes used in the treatment of “Osteoporosis”.
Commonly Asked Questions.
Give functions of thyroid hormones.
Write a short note on Synthesis Storage & Release of Thyroid Hormones.
Write a note on disorders of Thyroid gland.