Thyroid Hormones Flashcards
What hormones does the thyroid produce?
- thyroxine (T4)
- triodothyronine (T3)
What are the overall functions of T3 and T4?
- increase rate of basal oxygen use, basal metabolism, and rate of heat production
- modulate delivery of substrates and oxygen by cardiovascular and respiratory systems to sustain metabolic rate
Thyroid hormone levels change based on what?
- energy need
- calorie supply
- environmental temperature
What do you need for thyroid hormone synthesis?
- 2 tyrosines
- iodine
What is the major thyroid hormone produced?
thyroxine (T4)
What is the biologically active thyroid hormone secreted in lesser quantities?
triiodothyonine (T3)
What enzyme converts T4 to T3?
5’ deiodinase
What is the alternative product of T4 (inactive form of T3) that is produced when less thyroid hormone action is needed?
Reverse T3
The parafollicular cells (C cells) of the thyroid gland secrete what?
calcitonin
Under stimulation, colloidal thyroid hormone is absorbed where?
into follicular cells
How is iodine supplied for thyroid hormone synthesis?
supplied by diet via iodide
Thyriod hormones are stored where until stimulated to secrete?
extracellularly in follicular lumen
What is the ratio of T4:T3 within the thyroid gland?
10:1
What is the Wolff-Chaikoff effect?
transient reduction in thyroid hormone levels cause by ingestion of a large amount of iodine and lasts about 10 days
Excessively high levels of dietary iodide (like in the Wolff-Chaikoff effect) will suppress the activity of what?
2 Na/1 I pump
Thyroglobulin is stored in follicles as what until stimulation for release of thyroid hormones?
colloid
When stimulus of thyroid gland is received (thyroid stimulating hormone), what happens?
- retrieval of thyroglobulin from follicle lumen into endocrine cell via endocytosis
- lysosomes fuse with colloid droplet and release T3, T4, free MIT, and free DIT from thyroglobulin
- T3 and T4 enter blood
How are thyroglobulins retrieved from follicle lumen and into endocrine cell via endocytosis?
cell membrane forms pseudopods which engulf a pocket of colloid
What happens to MIT and DIT when thyroid hormones are secreted into circulation?
rapidly deiodinated by deiodinase to conserve iodide for recylcing
What are the effects of hepatic failure on circulating thyroid hormones?
- decreased liver protein synthesis
- decreased thyroxine binding globulin
- increase in circulating thyroid hormone
- decrease in hormone synthesis
What proteins does circulating T3 and T4 bind to?
- thyroxine binding globulin
- transthyretin
- albumin
What conditions cause sick euthyroid syndrome?
- fasting/starvation
- stress (cortisol)
- catabolic diseases - cancer
- hepatic diseases
- renal diseases
What enzyme is downregulated in sick euthyroid syndrome?
5’ deiodinase
Sick euthyroid syndrome is due to dysregulation of what?
thyrotropic feedback control
Describe the regulation of the thyroid gland.
- thyrotropin-releasing hormone is released from hypothalamus
- causes release of thyroid stimulating hormone from anterior pituitary
- causes release of T4 and T3 from thyriod gland
- negative feedback of T4 and T3 to anterior pituitary and hypothalamus
Since T4 cannot cause negative feedback itself, what must occur?
it has to be converted to T3 via 5’ deiodinase since T3 is the active form
Thyroid releasing hormone is a _____ hormone.
tripeptide
Thyroid stimulating hormone is a ____ hormone with ____ subunits.
glycoprotein, 2
What are the biologically active sites of TSH?
beta subunit
Describe the steps in the synthesis of thyroid hormones.
- need to synthesize protein molecule called thyroglobulin using tyrosine
- iodide transported from blood into follicular epithelial cells via 2 Na+ / 1 symporter
- iodide transported across apoical membrane into colloid of follicle via pendrin and oxidized to iodine via thyroid peroxidase
- iodine incorporated into tyrosines at specific sites in thyroglobulin via thyroid peroxidase
- coupling reaction – results in MIT and DIT whcih remain attached to thyroglobulin until stimulus for secretion
Negative feedback exerted predinantly at the ______ to decrease TSH.
anterior pituitary
What enzyme converts T4 to T3?
5’ deiodinase
What enzyme converts T4 to rT3?
5 deiodinase
What are the major sites of degradation of thyroid hormones?
- liver
- kidneys
- skeletal muscle
What hormones provide tonic inhibition for TSH release?
- dopamine
- somatostatin
How does T4 enter cells?
facilitated diffusion
T4 is deiodinated to T3 where?
inside cells
T3 binds the receptors within what of the cell?
nucleus
Why does T3 bind to the receptor within the nucleus of the cell?
- T3 receptor complex binds to thyroid regulatory element on DNA to stimulate transcription of genes
What are the effects of thyroid hormones on basal metabolic rate and metabolism?
- increased oxygen consumption and increased Na/K ATPase
- increased cardiac output and pulse pressure
- increased thermogenesis
- increase supply of substrates for energy production
- increase appetite, gut absorption, motility
- increase mobilization of carbs and fat
Protein synthesis vs degradation is dependent on thyroid hormone levels. Explain.
- if thyroid hormone levels are normal then protein synthesis predominates
- if thyroid hormone levels are high then protein degradation predominates to use amino acids for energy
Throid hormones act with what to promote growth of bone, cartilage, teeth, epidermis, and hair?
- growth hormone
- IGFs
What effects do thyroid hormones have on the central nervous system?
- increases blood flow and glucose metabolism in the brain
- enhances peripheral nerve reflexes
What effects do thyroid hormones have on the autonomic nervous system?
interact with sympathetic nervous system to have similar effects on increasing cardiac output, heat production, etc
What is hypoerthyroidism?
overactive thyroid causing excess thyroxine
If hyperthyroidism is in thyroid gland, TSH is high or low?
low
If hyperthyroidism is in hypothalamus or pituitary, TSH is high or low?
high
What is hypothyroidism?
underactive thyroid causing low thyroid hormones
If hypothyroidism is in thyroid gland, TSH is high or low?
high
If hypothyroidism is in hypothalamus or pituitary gland, TSH is high or low?
low
