Flashcards in Unit 6 - Thyroid Physiology Deck (40)
thyroid gland structure and function
-what makes it unqiue as an endocrine gland?
regulates vertebrate growth, development, and metabolism
-bilobulated, large (10-25 g), and stores large amounts of hormones
-only endocrine gland that is easily palpated during routine exam
-tremendous potential for growth (goiters weigh 100s g)
-highly vascularized (blood flow 4-6 mL/min/g)
-attached to trachea by loose CT
-made of numerous follicles (100-300 micrometers)
what are thyrocytes? what do they contain?
follicular epithelial cells (AKA thyrocytes) that are sites of thyroid hormone synthesis and release
-TH is stored with thyroglobulin (TG) in colloid (gelatinous inner area of follicles)
-has scattered parafollicular C-cells (sites of calcitonin synthesis/release)
-has fibroblasts, lymphocytes, adipocytes, and endothelial cells lining
how are T3 and T4 made?
made from tyrosine on thyroglobulin, and need iodide (I-) from diet
-preferential synthesis of T4 (prohormone), but T3 is biologically active
-rT3 is also made, and biologically inactive
what makes thyroid hormones unique?
they are the only hormones that need iodine
-most iodide is stored in thyroid gland while associated with thyroglobulin
-the rest is in target tissues or in circulation as organic/inorganic
what does TH bind to in bloodstream?
TBG; thyroid binding globulin
-not as commonly transthyretin and albumin
what is needed to stay in thyroid balance?
need to ingest adequate amounts of dietary iodide (70-300 ug/day)
-I- is concentrated in thyroid gland by specific transport PRO (2 Na+/I- symport) that uses inwardly-directed electrochemical Na+ gradient as driving force
--not a "pump" and doesn't use ATP directly
how does thyroid gland "autoregulate"?
autoregulates iodide transport according to needs
-if dietary iodide is low, it concentrates more
-if I- is high, it concentrates less
what does chronic iodide deficiency lead to? where is this true/
a form of hypothyroidism that is corrected by providing adequate dietary iodide
-found in "land-locked" mountainous areas outside USA
what is the "end" of TH?
-iodide is excreted in urine and feces
6 steps of synthesis and storage of thyroid hormones
1. in ER, thyroglobulin (TG) molecules are produced and packaged in vesicles by golgi, and exocytosed into lumen of follicle
2. I- enters thyrocyte via basolateral Na+/I- cotransporters (iodide trap), and exits apical side via I-/Cl- antiporters
3. in follicular lumen, I- is oxidized to iodine by thyroid peroxidase, and substituted for H+ on benzene ring of tyrosine residues of TG
4. binding 1 iodine = MIT, while 2 = DIT (organification)
-thyroid peroxidase catalyzes coupling of DIT to DIT (T4) or MIT (T3) that remain attached to TG
5. mature TG (with MIT > DIT > T4 > T3) are endocytosed back to follicular cell, and stored as colloid until secreted
6. colloid proteolysis is stimulated by TSH, and constituent molecules released
-DIT/MIT reenter synthetic pool
-T3/4 exit basolateral membrane to blood
what is diodinase?
in follicular cell of thyroid
-breaks MIT and DIT into I- to be recycled
-rarely deficient, and can be found in different cell types with different jobs
what is the main TH secreted?
7% T3 and rT3
-most of these are derived from T4
how much of TH is "free"?
T4 is 0.03%, T3 is 0.3%
-it needs to be in this form to enter target tissues and bind to TH receptors in nucleus
how are THs metabolized in tissues (like liver/kidney?)
via 5' peripheral deiodinases
-convert T4 to T3 or rT3
-if further deiodinate T3, will make inactive organic compounds
what toes propyltiouracil (PTU) do?
inhibits deiodinases that convert T4 to T3
-given to inhibit follicular cells in hyperthyroidism
-will take a while to get relief, so give beta-blockers for tachycardia as well
why is T4 given instead of T3 for treatment of hypothyroidism?
has longer half-life and greater stability
-slower onsets and longer duration of action
-T4 will convert to T3, which acts 4x as rapidly
how do TH enter cell and exert effect?
unknown method of entry
-T4 usually converted to T3 by cytoplasmic 5'/3'-monodeiodinase, so cytoplasmic levels of T3 = T4
-these bind to thyroid hormone receptor (THR) bound to retinoid X receptor (RXR), which are bound to nuclear DNA at thyroid response elements in promoter regions of genes regulated by TH for transcription
where are TH receptors expressed? what are their effects?
expressed in virtually all tissues of the body
-regulate metabolism of CHO, PRO, and lipids by way of:
--gluconeogenic and respiratory enzymes
--myosin heavy chain
-overall increase metabolic rate and O2 consumption
what are TH effects on
1. act synergistically with GH to promote bone formation, ossification, fusion of bone plates, and maturation
2. essential for development in perinatal period
3. increased CO2 output
4. increased CO
5. increased urea excretion and renal function
what happens to bone in hypothyroidism?
bone age is less than chronological age
-excessive replacement therapy with thyroxine can lead to bone loss and osteoporosis
what does TH deficiency do in infants? how is this treated?
mental retardation (cretinism) and growth retardation
-growth fixed with treatment of T4 (thyroxine)
-mental only fixed if initiated shortly after birth
effects of TH on BMR, metabolism, and cardiovascular
-more Na/K ATPase, O2 consumption, heat production
-more glucose absorption, glycogenolysis, gluconeogenesis, lipolysis, and net PRO catabolism
what does complete lack of TH secretion VS extreme excess secretion do to BMR?
lack: BMR falls to 40-50% below normal
excess: BMR rises 60-100% above normal
what does hypothyroidism do to serum cholesterol?
increased serum cholesterol will increase risk for atherosclerosis (LDL receptor expression and cholesterol excretion in bile are decreased)
-women who take T4 must be careful during menopause b/c of osteoporosis
what does hperthyroidism do to sympathetic system?
increased expression of beta-adrenergic receptors leads to enhanced sensitivity to circulating E and NE
-TH interact with SNS in unknown ways, so antagonists are effective in treating it
how is thyroid function regulated?
1. thyrotropin-releasing hormone (TRH) is tripeptide hypothlamic releasing factor that stimulates TSH release by activating GPCR linked to PLC, to generate IP3 and mobilize intracellular Ca++
2. TSH stimulates TH synthesis via GPCR to AC that increases cAMP
3. T3 (especially) and T4 act on target tissues for negative feedback on anterior pituitary and hypothalamus
4. dopamine and somatostatin exert inhibitory effects on TSH release
where is the defect in primary hyperthyroidism?
antibodies VS TSH receptors make it constantly on to make high T4 and T3
-strong negative feedback to hypothalamus (low TRH) and anterior pituitary (low TSH)
where is the defect in secondary hyperthyroidism?
tumor in AP constantly makes TSH
-high TSH makes high T4 and T3
-strong negative feedback to hypothalamus (low TRH) and AP (but can't do anything b/c tumor)
symptoms of hyperthyroidism
increased BMR (30-60%), tachycardia, sweating, heat intolerance, nervousness, muscle wasting, weakness, tremor, difficulty sleeping, changes in hair growth and skin texture