The thyroid gland Flashcards
What is abnormal growth of thyroid gland called? how large is it?
goiter - can be about 100g instead of 20g
Describe the chemical structure of the thyroid hormones
thyroid produces 3 hormones:
- T3
- T4
- Calcitonin
T3 = active form of T4, released as T4 then activated to T3
Where is T3/T4 and calcitonin produced?
T3/T4 in follicular cells (tyrosine + iodine)
calcitonin: produced in parafollicular cells - Ca2+ metabolism
Describe the synthesis of T3 & T4
transport of iodide into epithelial cells, along 2 Na+ against conc. (ATP) (sodium-iodide symporter)
tyrosine rich protein (thyroglobulin - Tg) made in epithelial cells
exocytosis secretion of thyroglobulin into lumen of follicle
oxidation of iodide to produce an iodinating species (I- to I0) OIL
iodination of side chains of tyrosine residues form: MIT & DIT
coupling of DIT + MIT/DIT forms T3/T4 in thyroglobulin
Describe the location & structure of the thyroid gland
ductless alveolar gland at the base of the neck - about 20g
gland wraps around trachea below cricoid cartilage
has 2 lobes connected by isthmus
3 arteries & veins: superior, middle & anterior
How is thyroid hormone stored?
T3 & T4 are stored extracellularly of epithelial cell, within the lumen of the follicle, part of thyroglobulin
How is thyroid hormone secreted?
determined by TSH from pituitary gland
hypothalamus –> TRH –> pituitary –> TSH –> thyroid –> T3/T4
thyroglobulin takes into epithelial cells via endocytosis from follicle lumen
proteolytic cleavage of thyroglobulin releases T3&T4 diffuse to circulation
How is thyroid hormone transported
bound to proteins (TBG) thyroxine binding globulin & albumin
What happens to thyroid production in pregnancy?
oestrogen increase the synthesis of TBG, therefore decrease T3/T4 as more bound to the transport protein
therefore decrease neg feedback, increasing TRH & TSH, stimulating thyroid to produce more T3 & T4
so free T3 & T4 levels return to normal, although total T3/T4 increases
Describe how the activity of the thyroid gland is controlled
dorsomedial nucleus of hypothalamus –> TRH –> thyrotrophs of ANTerior pituitary –> TSH –> follicular cells of thyroid –> T3/T4
When does TRH increase? where does it travel to reach to anterior pituitary?
tripeptide structure
when increase in stress and decrease in temperature
travel via the hypothalamic pituitary portal system
Explain a little about TSH - structure, when it’s released, what effects it has
glycoprotein: 2 non-covalently linked sub units (a & B), released in low amplitude pulses following circadian rhythm: increase at night, decrease in morning
interacts with receptors on follicle cell surface: stimulates synthesis & secretory T3/T4
has trophic effects on gland: increase vascularity, increase in size & number of follicle cells - can lead to enlarged thyroid (goitre) - over / under active
Describe the effects of thyroid hormones (T3/T4) on cells as a whole
increase metabolic rate: increase glucose uptake & metabolism (glycogenolysis & gluconeogenesis)
stimulate mobilisation & oxidation of fatty acids (lipolysis)
stimulate protein metabolism (proteolysis)
increase in number & size of mitochondria - more ATP
Describe the effects of thyroid hormones (T3/T4) on body as a whole
- increase in BMR
- increase in heat production (UCPs) - uncoupler
- increase in oxygen consumption (ETC)
- increase protein synthesis: promote growth & development
- increase responsiveness of tissues to SNS
Describe the effects of over secretion of thyroid hormones
Signs & symptoms
Hyperthyroidism - may mature earlier
- heat intolerance, increase perspiration, warm moist hands
- weight loss - proteolysis, lipolysis
- tachycardia - increase cardiac output
- increase bowel movements - increase appetite
- nervousness, irritability & emotional liability
- hyper-reflexive: tremor of outstretched hand
- eye signs: exophthalamas (bulging of eyes anterior out of orbit)
