Thyroid and Parathyroid Glands Flashcards
What is the anatomy of the thyroid gland?
- 2 lateral lobes (left and right)
- isthmus connecting loves transversely
Location:
- located between C5-T1 levels
- overlies cricoid cartilage and superior tracheal rings
What is the internal structure of the thyroid gland?
Follicles:
- thyroid composed of hollow spherical follicles
- walls of follicles made up of follicular cells in a basement membrane
Follicular cells:
- make up walls of follicles in thyroid
- secrete thyroglobulin (a glycoprotein that is the precursor of T3 and T4 hormones)
- glycoprotein stored in follicular lumen, suspended in a viscous gel called colloid
Parafollicular cells (C cells): - secrete calcitonin (effects storage of calcium in bones)
What is the arterial and venous supply of the thyroid?
Highly vascular
arterial:
- superior thyroid artery (first branch of external carotid artery)
- inferior thyroid artery (from thyrocervical trunk of subclavian artery)
venous;
- superior and middle thyroid veins draining into internal jugular
- inferior thyroid vein draining into brachiocephalic vein
What are the thyroid hormones?
T3:
- more biologically active
- has a greater affinity for TH receptors
T4:
- less biologically active
- converted to T3 upon entry into cells
Actions:
- increase gene transcription associated with glucose oxidation (increases BMR and body heat production)
- catecholamine like effects (maintain BP and cardiac function, activate adrenergic receptors in blood vessels)
- regulation of normal development and functioning (nervous, MSK, digestive, reproductive, integumentary systems)
What is the process governing the release of thyroid hormones?
- Hypothalmaus releases TRH (thyrotrophin releasing hormone)
- Anterior pituitary releases TSH (thyroid stimulating hormone)
- Thyroid: TSH binds to receptors on thyroid follicular cells
- causes release of stored T3 and T4 hormones
- causes synthesis of more thyroglobulin into T3 and T4 to store in colloid - T3 and T4 circulate bound to TBGs (thyroxine-binding globulins) and albumin in bloodstream
Which hormone is the precursor to thyroid hormones?
Thyroglobulin
- synthesized into T3 and T4 to be stored in colloid of follicular cells
Which substances do T3 and T4 bind to in the bloodstream?
TBGs (thyroxine-binding globlins) and albumin
Which method of activation do thyroid hormones use?
Direct gene activation
- bind directly to thyroid hormone receptors fixed to DNA of target cells and alter mRNA transcription
What is the process of synthesis for T3 and T4?
- Thyrogloblin the precursor hormone (produced by follicular cells in thyroid gland)
- Iodine actively transported into follicular lumen
- iodine attaches to a tyrosine molecule (process requires the enzyme thyroid peroxidase)
- T1 (monoidothyronine) and T2 (diiodothyronine) are linked together to form T3 and T4 hormones
What are the parathyroid glands?
- small endocrine glands on posterior thyroid
- most people have 4 parathyroid glands
- secrete PTH (parathyroid hormone) which increases plasma levels of calcium
What are the actions of PTH?
PTH:
- parathyroid hormone (released by parathyroid glands)
- on bones:
- stimulates release of calcium from calcium phosphate cells in bone
- increases bone destruction
- decreases formation of new bone - on kidneys:
- reduces calcium excretion in urine
- stimulates production of vitamin D (necessary to absorb calcium) - on small intestines:
- increases calcium absorption (via effects on vitamin D metabolism)
What is the role of calcium in the body?
Storage:
- 99% of body’s calcium is stored in bone in the form of calcium phosphate cells
Functions:
- provides strength to skeleton
- acts as a dynamic calcium reservoir (ionized calcium can be released into the ECF)
- enhances neuromuscular excitability
- intracellular signalling
- blood clotting
- secretory activity
