Thyroid hormone synthesis and action Flashcards
What is the secretion of parafollicular/C-cells?
Calcitonin - lowers blood Ca levels
Describe the structure of the thyroid gland
Thyroid follicles
Single layer of epithelial cells (follicular cells)
Lumen filled with protein rich colloid
Microvilli on surface
C-cells/parafollicular cells
What hormones are produced by follicular cells of the thyroid gland?
Derived from tyrosine:
- Triiodothyronine (T3) - 10%
- Thyroxine (T4) - 90%
T3 is more biologically active - T4 converted to T3 in tissues
Describe the synthesis of thyroid hormones
Iodine transported into follicular cells via secondary active transport
Follicular cells produce thyroglobulin - rich in tyrosine AA
Iodine gets stuck on tyrosine in thyroglobulin in the colloid => T3 and T4 stored in colloid bound to thyroglobulin
Describe iodine uptake in thyroid hormone synthesis
I- circulates in blood
Transported into follicular cells by secondary active transport:
- Na/I- symport in basal membrane stimulated by TSH
Describe iodination in thyroid hormone synthesis
Iodide ions are oxidised to iodine - catalysed by thyroperoxidase
Iodine is added to tyrosine in thyroglobulin - catalysed by iodinase enzyme
Describe the secretion of thyroid hormones
Thyroglobulin re-enters follicular cell from colloid via endocytosis
Intracellular vesicles fuse with lysosomes
Lysosomal enzymes split thyroid hormones from thyroglobulin
Hormones diffuse across basal plasma membrane into interstitium then into blood
Describe how thyroid hormone is transported in the blood
Lipid-soluble
Bound to thyroid-binding globulin
describe the metabolism of thyroid hormones
90% Thyroxine (T4) deiodinated to free triiodothyronine (T3) in liver and kidneys and within cells
Remaining T4 is deiodinated to reverse T3 (rT3)
Free T3 is biologically active
rT3 is inactive - slows metabolism
How does the body adapt to starvation via thyroid hormones?
Thyroid hormones increase BMR (basal metabolic rate) so during starvation or illness:
- lower TSH from pit gland => less T3 and T4 production
- alters deiodination => more rT3
How are thyroid hormones catabolised?
Deiodinated into diiodotyrosine and monoiodotyrosine in liver and kidneys
Iodine is recycles or excreted via urine
Define euthyroid, hyperthyroidism and hypothyroidism
Euthyroid = normal thyroid function
Hyperthyroidism = increased function
Hypothyroidism = decreased function
Describe the regulation of thyroid hormones
TRH (thyrotropin releasing hormone) from hypothalamus stimulates anterior pit gland to secrete TSH/thyrotropin (thyroid-stimulating hormone) => acts on thyroid gland
what are the actions of TSH (thyroid stimulating hormone)?
Increased endocytosis and proteolysis of thyroglobulin from colloid
Increased activity of Na+/I- symport
Increased iodination of tyrosine (increased activity of iodinase and thyroid peroxidase enzymes)
Increased size and secretory activity of thyroid follicular cells
Increased number of follicular cells
What is goitre?
enlargement of thyroid gland due to TSH overstimulation
Cats with hyperthyroidism
