The Thyroid Gland Flashcards
What are the two thyroid hormones?
T3: triiodothyronine
T4: thyroxine
What are the two cell types which make up the thyroid gland? What are their function?
C (clear) cells - secrete calcitonin
Follicular cells - support thyroid hormone synthesis and surround hollow follicles
What are thyroid follicles?
Spherical structures whose walls are made of follicular cells. The centre of the follicle is filled with colloid - a sticky glycoprotein matrix
What do follicular cells manufacture? what happens to these products once created?
The enzymes that make thyroid hormones as well as thyroglobulin. a large protein rich in tyrosine residues
The enzymes and thyroglobulin are packaged into vesicles and exported from the follicular cells into the colloid
What do follicular cells concentrate and transport into the colloid? What does this become?
It actively concentrates iodine from the plasma and transports it into the colloid where it combines with tyrosine residues to from the thyroid hormones
How does iodine enter the follicular cells from the plasma? Where does the iodine go from here and how?
Via a Na+/I_ transport system. Th coupling Na+ allows the cells to take up iodine against a concentration gradient
The iodine is then transported into the colloid via the pendrin transporter
What govern the release of TH from the colloid into the plasma?
TSH released from the pituitary. IT stimulates the follicular cells to endocytose colloidal thyroglobulin
What is formed when one iodine is added to tyrosine?
MIT - monoiodotyrosine
What is formed when two iodines are added to tyrosine?
DIT - diiodotyrosine
What do MIT + DIT form?
T3
What do DIT + DIT form?
T4 (thyroxine)
What is the enzyme present on the colloidal side of the cells of the thyroid which catalyses the addition of iodine to tyrosine residues in the thyroglobulin?
Thyroid peroxidase
Are T3 and/or T4 lipid soluble?
Both are lipid soluble
What is the main plasma protein to which T3 and T4 bind to in the plasma?
Thyroxine-binding globulin
What proportion of T3 and T4 is in the bound state with plasma proteins?
99.8%
Does T3 or T4 have a longer half-life than the other? Why?
T4 has a longer half life than T3 - around 6 days vs 1 day
Because the affinity of thyroxine-binding hormone is higher for T4 than T3
What are two classes of chemicals which are inhibitory to thyroid hormone secretion?
Glucocorticoids
Somatostatin
Decribe the pathway of the secretion of TH
Tonic release in the hypothalamus triggers TRH release to the anterior pituitary, which releases TSH into the bloodstream, which acts upon the thyroid gland to stimulate T4 and T3 (T3 differs by one less iodine and is the active form of T4) and this exerts the systemic metabolic effects such as raising metabolic rate and producing heat, uncoupling oxidative metabolism in mitochondria, playing a critical role in growth and is involved in foetal brain development
Give some causes of hyperthyroidism
Graves disease - antibodies are produced which bind and mimic TSH, continually activating the thyroid gland. The increased production of TH switches off TSH release from the anterior pituitary so the TSH in the plasma is very low. The thyroid gland may be 2-3x normal size due to hyperplasia
Thyroid adenoma - hormone-secreting thyroid tumour (rare)
What are some symptoms of hyperthyroidism?
Heat intolerance/weight loss
Muscle weakness
Hyper excitable reflexes and psychological disturbances
Increased HR, contractile force, output -> cardiac failure
What are some causes of hypothyroidism?
Hashimoto’s Disease - autoimmune attack of the thyroid gland
Deficiency in dietary iodine - only 50mg per year required but many areas of world soil has insufficient quantities
Idiopathic - no known cause, may be linked to thyroiditis
What are symptoms of hypothyroidism?
Cold intolerance/weight gain
Brittle nails/thin skin
Slow speech/reflexes, fatigue
Slow heart rate/weaker pulse
What is the manifesto that after often accompanies hypo and hyperthyroidism?
Goiter
