Thyroid Flashcards
Thyroid hormone provides long term regulation of what?
Core body temperature
What does the thyroid hormone act on?
Metabolism! (not thermoregulatory mechanisms)
What does the hypothalamic -pituitary-thyroid axis do?
Regulates thyroid hormone release from the thyroid gland
How does the HPT regulate thyroid hormone release?
- PVN releases thyrotropin releasing hormone (TRH) into portal capillaries and the anterior pituitary
- Anterior pituitary release thyroid stimulating hormone (TSH) into general circulation
- Thyroid gland cells respond to TSH by releasing thyroid hormone into circulation
Where is the thyroid gland located?
Between the thyroid cartilage down to the 6th tracheal cartilage ring
What is a follicle?
- functional unit of thyroid
- having simple cuboidal cells incorporating iodine and synthesizing thyroglobulin
- having a colloid interior where T3 and T4 are synthesized and stored with thyroglobulin
Explain T3 and T4 synthesis
- combining tyrosine, iodine, and thyroglobulin
- dietary iodide binds to tyrosines of thyroglobulin forming MIT and DIT
- MIT coupled with DIT forms T3 and T4
What does TSH do?
released thyroid hormones from follicular cells by endocytosis thru pseudopodia
How does TSH release thyroid hormones?
TSH activates cAMP which does:
- endocytosis of colloid
- thyroidal uptake of sm. molecules
- intracellular trans. of proteins
- most steps of thyroid hormone storage, synthesis, and release
- removing the “I” from MIT and DIT
What are the general functions of T3 and T4?
- T3 affects every organ
- acting as a modulator of cell function
- essential in development
What is the role of T3 and T4 in maturation?
skeletal and nervous system development
What is cretinism?
early thyroid disease
- abnormal bone and CNS development
- stunted growth, malformation of facial bones, mental retardation
What is the T3 mechanism of action?
- T3/4 cross membrane via ATP-ase transporter
- T4 converts to T3 via iodothyronine deiodinase
What does T3 enhance?
- BMR
- oxidative metabolism (except in brain, testes and spleen)
- increased oxygen consumption and greater ATP utilization
- mitochondrial activity in skeletal muscle, heart, kidney, and liver
How does T3 act on metabolism?
- enhances BMR via increased O2 intake
- increases overall activity in muscles, organs, fat and thermoregulation via stored food and O2
- facilitates hormonal actions over glucose, lipids, and protein metabolism
- increases CO by up regulation of beta receptors
What are activated in a response to cold and strenuous exertion?
SNS and adrenal medulla
What is permissive action?
T3 increases the response to catecholamines, and maintains a metabolic rate for generation and mobilization of substrates involved in adrenal response
What do catecholamines enhance?
- T4–>T3 conversion (select tissues)
- increase glucose formation and enhance metabolic turnover
What is another action of T3?
- uncoupling of ATP synthesis in catabolism, energy is released as heat
- increases ion channel permeability, enhancing Na/K pumping and ATP hydrolysis (releases heat)
Explain the negative feedback control of T3,T4 secretion
T3 and T4 determined by T3 feedback inhibition of synthesis and release of TRH and TSH
TRH and TSH expression is ______ related to T3 and T4
inversely
- T3 down regulates TRH and TSH synthesis
Describe T3 feedback inhibition in the pituitary gland
T3 feedback inhibits TSH secretion by reducing anterior pituitary’s sensitivity to TRH (downgrades its synthesis and release)
What does feedback on the HTP axis include?
- T3,T4 conversion
- energy state (food intake/metabolism)
- temperature
How do you increase thermogenesis?
manipulate thyroid feedback sensitivity
Cold increases the release of _____ and increases ______ production.
TRH…..T3 and T4
How does cold exposure affect adrenal release?
Epinephrine release is stimulated and lowers responsiveness of PVN cells to T3
What is hyperthyroidism?
increased synthesis of T3 and T4
What are some causes of hyperthyroidism?
- reduced sensitivity of feedback
- antibodies to thyroid gland release excess T3 and T4
- -> excess T3 and 4 inhibits TSH release
What are symptoms of hyperthyroidism?
- goiter (hyperplasia of thyroid cells)
- heat intolerance
- weight loss
- exophthalmos (protrusion of eyes due to lid lag)
- nervousness, tremor
- irritability, insomnia
- tachycardia, arrhythmias
What is hypothyroidism?
- decreased synthesis of T3 and T4
- decreased conversion of T4 to T3
What are some causes of hypothyroidism?
- autoimmune destruction (often with thyroiditis)
- iodine deficiency
- dietary goitrogens (turnips, cabbages)
What are the results of decreased T3, T4 secretion?
- less hypothalamus and pituitary negative feedback
- T3 and T4 levels are low while TSH levels are high
- increased colloid due to increased TSH activity
What are some symptoms of hypothyroidism?
- colloid goiter
- cold intolerance
- fatigue, sluggishness, slow speech, lack of initiative
- delayed tendon reflexes
- bradycardia
- high cholesterol
- hair loss, dry and corse skin, husky voice
- enlarged liver, kidney, and tongue
- myxedema (non-pitting edema)
