Thyroid gland Flashcards
describe the structure of the thyroid gland
2 lobes joined by the isthmus
describe the embryological development of thyroid gland
- originates from foramen caecum
- thyroid diverticulum grows down > thyroglossal duct
- as thyroid descends the thyroglossal duct breaks down
describe the microscopic structure of thyroid glands
- thyroid follicles - follicular cells in spheres
- filled with colloid - deposition of thyroglobin
what do thyroid follicular cells produce?
thyroid hormone
what do parafollicular cells produce?
calcitonin
what are the principal cells of the parathyroid gland?
what do they produce?
chief cells
parathyroid hormone
how are thyroid hormones formed?
2 iodinated tyrosine molecules are linked together with iodine at 3 or 4 positions on aromatic rings
what is needed to make T3?
monoiodotyrosine + diiodotyrosine > triiodotyrosine
what is needed to make T4?
diiodotyrosine + diiodotyrosine > tetraiodotyrosine/thyroxine
what is the role of thyroglobulin in thyroid hormone formation?
acts as a scaffold on which thyroid hormones are produced
what is thyroid peroxidase?
membrane bound enzyme that regulates thyroid hormone synthesis
what does thyroid peroxidase regulate to thyroid hormone synthesis?
- oxidation of iodide to iodine
- addition of iodine to tyrosine acceptor residues on thyroglobulin
- coupling of MIT/DIT to generate thyroid hormones
outline the absorption of dietary iodine in the thyroid gland
- dietary iodine reduced to iodide
- iodide absorbed by small intestines
- iodide taken up from blood by thyroid epithelial cells via Na/I- symport
sources of iodine
dairy products
meat
vegetables
eggs
grains
where is T4 converted to T3?
liver + kidneys
how are thyroid hormones transported in the blood?
lipid soluble
bound to thyroxine-binding globulin
what thyroid hormone is more biologically active?
T3
Actions of thyroid hormones
- increased BMR
- affects growth in children (bone + CNS development)
- increased HR + CO
- increase thermogenesis
- important for normal reproductive function
describe the structure of TSH
glycoprotein hormone
composed of 2 non-covalently bound subunits aB
what does TSH stimulate?
- iodide uptake
- oxidation of iodide to iodine
- thyroglobulin synthesis
- thyroglobulin iodination
- colloid pinocytosis into cells
- proteolysis of thyroglobulin
- cell metabolism + growth
how do thyroid hormones increase BMR + heat production?
- increase number + size of mitochondria
- stimulate synthesis of enzymes in respiratory chain
what metabolic pathways are stimulated by thyroid hormone?
- lipid metabolism - lipolysis + B-oxidation
- carb metabolism - gluconeogenesis, glycogenolysis, insulin dependent entry of glucose into cells
effect of thyroid hormones on CVS
- increases hearts responsiveness to catecholamines:
- increase CO (HR and force of contraction) + peripheral vasodilation
effect of thyroid hormones on nervous system
- increased development of neurones
- increased myelination of nerves
What affect would lack of dietary iodine have on?
- T3+4
- TSH
- goitre present?
- decreased T3+4
- increased TSH
- yes, goitre present
Blood supply to the thyroid
Superior + inferior thyroid artery
Venous drainage of the thyroid
- superior + middle thyroid veins drain into internal jugular vein
- inferior thyroid vein: drains into left brachiocephalic vein
Origin of the superior thyroid artery
Branches from external carotid artery
Origin of the inferior thyroid artery
subclavian artery > thyrocervical trunk > inferior thyroid artery
