Thyroid Flashcards
where is the thyroid gland?
at base of neck (butterfly shape)
what are the two main classes of thyroid hormones?
- T3 and T4 thyroid hormones
2. calcitonin
is T3 or T4 more active on THR?
T3
what are T3 vs T4 aka?
T3: triiodothryonine
T4: thyroxine
what is the highest-circulating thyroid hormone?
T4T
What is the HPT axis for thyroid hormones?
TRH released from hypothalamus stimulates ant pit to release TSH which stimulates thyroid (follicular cells) to release T3/T4 (neg feedback on hypo and ant pit)
what are T3 and T4 collectively called?
thyroid hormone
what are 3 physiological effects of thyroid hormone?
incr basal metabolic rate, sensitization to catecholamines (NA: incr HR, CO, breathing rate), growth and development
are thyroid hormones synthesized from free tyrosine molecules?
No, synthesized from precursor protein (thyroglobulin)
How many modified tyrosine molecules make up thyroid hormones?
2 (have iodine molecules)
What is the process of thyroid hormone synthesis?
tyrosines undergo enzymatic iodination/iodide undergoes organification into MIT (1 I2) or DIT (2 I2), iodinated tyrosines are enzymatically coupled into T3 (MIT + DIT) or T4 (2 DIT)
What position on the aromatic tyrosine ring are iodines added to?
3 and 5
what stimulates T3 or T4 synthesis?
TSH stimulation causes thyroglobulin endocytosis and processing into T3 or T4 (> released)
what is the basic unit of the thyroid gland? (3) and what occurs in each part?
apical side of follicle: iodination and coupling of thyroglobulin (lumen)
follicle cell: thyroglobulin processing
basolateral side: T3/4 released into bloodstream
how does the thyroid gland concentrate iodide into follicle lumen?
from bloodstream via Na/I cotransporter (basolateral, Na down, I up)
what is the thyroid hormone receptor?
intracellular (TF w/ TH binding)
how is the THR an exception to most intracellular receptors?
T3/4 are not lipid-soluble and require a transport protein to reach THR
what is THR bound to at rest (unactivated)?
thyroid response elements (TREs) on DNA and corepressors (homodimers)
what occurs to T4 when it enters the cell?
de-iodinated to T3 (enzyme)
what occurs when T3 binds to the THR?
recruitment of RXR (retinoic acid receptor) to form heterodimer (1 THR replaced) and coactivator binding (incr transcription)
what are the 4 most common causes of hypothyroidism?
iodine deficiency, autoimmunity towards thyroid (Hashimoto’s thyroiditis), developmental defect, inappropriate hormonal regulation (decr TRH/TSH)
what are 4 symptoms of hypothyroidism?
fatigue, weight gain (decr met rate), hypersensitivity to cold, bradycardia
how are thyroid diseases diagnosed? (2)
measure TSH (THR circ never high-acts directly on ant pit) or anti-TSH antibodies for hyperthyroidism
what is a cause and features of primary hypothyroidism?
cause: defect in thyroid function
feature: low T3/4, high TSH (low neg feedback)
what is a cause and features of secondary hypothyroidism?
cause: central defect (hypo/ant pit)
features: low TSH, low T3/4 (decr TSH)
what is a treatment for hypothyroidism?
hormone replacement
which hormone is used for hormone replacement in the treatment of hypothyroidism?
T4/thyroxine (main circ form)
what is the name of the drug prescribed for hypothyroidism?
levothyroxine
what are 2 most common causes of hyperthyroidism?
Grave’s disease (autoimmune response to TSHR-activates) and hyperplasia (thyroid adenoma, goiter-excessive growth)
what are 5 symptoms of hyperthyroidism?
sleep difficulty, heat intolerance, tachycardia, weight loss, tremor (incr met rate)
what’s the cause and features of Grave’s disease?
cause: thyroid activation by anti-TSHR Abs
features: high T3/4, low TSH (neg feedback)
whats the cause and features of thyroid hyperplasia?
cause: benign tumour in/on thyroid
features: high T3/4, low TSH (neg feedback)
whats the cause and features of secondary hyperthyroidism?
cause: central defect (incr TSH from ant pit)
features: incr TSH, high T3/4
whats the difference btwn Grave’s disease and Hashimoto’s thyroiditis?
both autoimmune
Grave’s: Abs stimulate TSHR
Hashimotos: Abs damage thyroid
what are 2 special diagnostic features of Grave’s disease?
exophthalmos (bulging eyes): autoimmune damgae to eye muscle/fibroblasts
goiter (swelling of neck): overactive thyroid (stim Abs)
t/f: goiter is only seen in Grave’s disease
false, also seen in hypothyroidism (iodine deficiency) if TSH incr (stim thyroid)
what is the surgical approach for hyperthyroidism and its drawbacks?
approach: part/all of thyroid is resected
drawbacks: require hormones replacement (hypothyroidism), disruption to parathyroid (Ca disturbances/hypoparathyroidism)
what is the radioactive iodine treatment approach for hyperthyroidism and its drawbacks?
approach: I^131 concentrates in thyroid, radiation destructs thyroid
drawbacks: hormone replacement, not for pregnancy/nursing (damage to infant)
what is the anti-thyroid drug (methimazole) approach for hyperthyroidism and its drawbacks?
approach: prevents synthesis of T3/4
drawbacks: diverse
what is the symptomatic B-blocker approach for hyperthyroidism and its drawbacks?
approach: B-blockers help tachycardia
drawbacks: doesn’t treat underlying cause of disease
how do thioamides (methimazole) treat hyperthyroidism?
inhibits thyroperoxidase enzyme (iodination/organification+coupling of thyroglobulin into colloid)
