Flashcards in Growth hormone and thyroid hormone Deck (41)
what processes does growth of the ovum to the adult involve?
what does this growth depend on?
-Multiple hormones regulate linear growth:
-GH, IGF-1 and IGF-2, insulin, TH, glucocorticoids, androgens and estrogens
-GH and IGF-1 have been implicated as the major determinants of growth in normal postuterine life
-Deficiencies (or excesses) of these can seriously affect normal growth and development
what is hyperplasia?
increase in number of cels
what is hypertrophy?
increase in size of cells
what is the principal endocrine regulator of growth and where is it secreted?
-growth hormone (GH)
-secreted somatotropin in the anterior pituitary
what does excessive or deficiency in GH secretion cause?
-Excessive GH secretion during childhood -> gigantism
-Deficiency of GH during childhood -> pituitary dwarfism
what does GH replacement in adults with GH deficiency lead to?
increased lean body mass and decreased body fat
what does excess of GH after puberty result in?
-Characterised by thickening of bones of the head, hands, feet, plus growth of skin, muscle, heart, liver and GI tract
How and when is GH secereted from somatotropin?
-Somatotrophs secrete GH in pulses
-Exercise, stress, high protein meals, and fasting also cause a rise in the mean GH level, where increased GH output results from an increase in frequency, rather than the amplitude, of pulses of GH that the somatotrophs secrete
what is GH secretion controlled by?
under hierarchal control from: GHRH - increase secretion of GH Into blood
Somatostatin- inhibits section of GH
what does GH trigger the secretion of?
IGF-1 from GH target tissues throughout the body- mediates many of the growth-promoting actions of GH
what does IGF-1 do in circulation?
produce its endocrine effects, largely reflects its hepatic synthesis
what way does IGF-1 synthesised in tissues such as muscle , cartilage and bone act?
paracrine or autocrine fashion to promote local tissue growth
what are the feedback systems of GH and IGF-1?
Both GH and IGF-1 negatively feedback on GH secretion by somatotrophs
what are acute metabolic effects of GH on muscle ,adipose tissue and liver?
-Stimulation of lipolysis in adipose tissue
-Inhibition of glucose uptake by muscle
-Stimulation of hepatic gluconeogenesis
-Termed anti-insulin or diabetogenic actions of GH
what are the long term effects of GH through IGF-1?
-GH promotes tissue growth by stimulating target tissues (incl. liver, kidney, muscle, cartilage and bone) to produce IGFs (peptide hormones)
-Liver produces most of the IGF-1 present in the circulation
-IGF-1 is the principal mediator of the growth-promoting action of GH
what else can promote growth?
-thyroid hormones - deficiency can cause dwarfism or cretinism
- sex steroids - oestrogen accelerates bone growth
-insulin - important in utero , severe deficiency leads to wasting of lean body mass
what do glucocorticoids do?
-In children, an excess of glucocorticoids inhibits growth (despite GH and IGF-1 levels being normal
- excess impairs tissue anabolism
what does longitudinal growth involve?
growth involves lengthening of the somatic tissues (incl. bone, muscle, tendons and skin) through a combination of tissue hyperplasia and hypertrophy
what role does leptin play in regulation of body mass?
-Protein made almost exclusively in adipocytes
-Crosses the BBB to modulate specific neurons in the arcuate nucleus of the hypothalamus that control feeding behaviour
-Plasma leptin levels mass of adipose tissue
-Long-term regulator of CNS feeding behaviour
what role does Ghrelin play in regulation of body mass?
-Promotes appetite – systemically administered ghrelin acutely increases food intake when given at physiological doses
-Synthesised by endocrine cells in the gastric mucosa in response to fasting
-Circulating levels are low in lean humans
-Acts via the arcuate nucleus of the hypothalamus
Describe thyroid hormones roles in growth.
The thyroid hormones:
Thyroxine (tetraiodothyronine or T4)
Act on multiple tissues and are essential for normal development, growth and metabolism
No cell membrane receptors exist for these hormones
Instead they act via nuclear receptors and regulate transcription of cell proteins
what are the follicles that make up the thyroid gland filled with?
what are follicle cells responsible for?
Outline the steps of synthesis of thyroid hormones.
1. increased iodine trapping
2. iodide leaves cell and secretes thyroglobulin
3. TSH also stimulates (step 2)
4. TSH stimulates conjugation of iodinated tyrosine to form T3 and T4 linked to thyroglobulin
5. endocytosis of iodinated thyroglobulin into follicular cells
6. proteolysis of thyroglobulin
7. secretion of T3 and T4 into circulation
which of the two thyroid hormones are more active?
what metabolises T4 to T3?
in circulation, what are both T3 and T4 highly bound to?
in terms of hormones which ones have a longer half live?
Steroid and thryoid hormones have longer half lives than peptide hormones