Lec 6 Thyroid Diagnostics Flashcards
What is structure of TSH?
pituitary glycoprotein w/ 2 alpha and 2 beta subunits
What is mutation associated with thyroid hot nodule?
constitutively active TSH receptor –> autonomous thyroid function
How can you confirm presence of thyroid hot nodule?
do radioiodine thyroid scan to show iodine uptake in nodule and suppressed uptake in rest of normal tissue
What are the steps in thyroid hormone synthesis and secretion?
- thyroid follicular cells concentrate iodide from extracellular space
- Na-I symporte brings I in
- high I permits organification of iodide into tyrosine residues on thyrogolobulin storage protein in follicular lumen
- condensation of iodinated tyrosine rings yields T4 and T3
- hydrolysis of thyroglobulin releases thyroid hormone into circulation
What is mech of antithyroid drugs?
inhibits thyroid hormone synthesis [organification] but do not release the stored hormone –> medication response is delayed by several weeks
What is difference in action/half life/etc T3 and T4?
T3 = higher potency and higher receptor affinity but shorter half life
What is problem with measuring dessicated thyroid level?
T3 spikes a few hours after does and then goes down; T4 relatively constant
pt can have high spike causing symptoms of hyperthyroid but at the time you measure them might show up with normal levels
What is euthyroid sick syndrome?
normal TSH but T3 looks really low b/c most of circulating T3 is from peripheral tissue deiodination of T4 [mostly in liver]
in pt who has chronic illness and starvation [cachexia] –> downregulated conversion to T3
What is T3 toxicosis?
when you are hyperhtyroid you tend to secrete more T3 compared to T4 –> disproportionate elevation of T3 relative to T4
can have high T3, normal T4, and low TSH and be hyperthyroid
What is significance of thyroxine binding globulin?
binds most T3 and T4 in circulation; if its high –> makes T3/T4 look high; if its low –> makes T3/T4 look low
What happens to free hormone level and total hormone level with altered hormone binding?
- free hormone level stays the same
- total hormone level changes
What are 5 causes of increased TBG?
- estrogen
- oral contraceptives
- estrogen replacement therapy
- pregnancy
- acute infectious hepatitis
What are 4 causes of decreased TBG?
- androgens
- nephrotic syndrome
- hypoproteinemia
- acromegaly
What is most sensitive test for thyroid dysfunction?
TSH
What determines clinical status of thyroid?
free T3/T4 levels
