2/14 Thyroid Testing/Pharm/Cancer - Wondisford

Question 1

TSH

Answer 1

single best test of thyroid fx

does NOT inform physician about degree of thyroid dysfx (long-linear relationship between TSH and freeT4)

**get a free T4 after abnormal TSH or if high clinical suspicion of thyroid disease

Question 2

what determines bioactivity of hormone at TSH receptor?

Answer 2

sugars on surface and their structure

in order of increasing bioactivity:

• central hypothyroidism
• euthyroid
• TRH treatment

Question 3

thyroid disorders

TSH vs free T4

Answer 3

lo TSH, lo T4 → central hypothyroidism

hi TSH, lo T4 → primary hypothyroidism

lo TSH, lo T4 → thyrotoxicosis hyperthyroidism & other

hi TSH, hi T4 → atypical thyrotoxicosis

Question 4

testing summary

Answer 4

Question 5

thyroid hormones

half life

half life reduction conditions

Answer 5

T4 (prohormone)

• half life: 7d
• reduced in thyrotoxicosis, P450 inducers

T3 (hormone)

• half life: 1d
• reduced in thyrotoxicosis, P450 inducers

Question 6

thyroid hormone replacement

Answer 6

replace in hypothyroid patients

NOT INDICATED/DETRIMENTAL: obesity, abnl vag bleeding, depression if thyroid levels normal

Question 7

synthetic T4

Answer 7

indicated for hypothyroid patients

long half life: 7d

once daily dosingt;’

Question 8

synthetic T3

Answer 8

ltd use

short halflife: 1d

multiple daily dosing

Question 9

Wolff Chaikoff Effect

Answer 9

too much iodide can block thyroid gland fx

• excess I inhibits coupling, presumably by blocking entry of DIT into second site

NOT a permanent effect

• chronic high I inhibits Na/I symporter → escape from W-D effect
  
  • if escape doesnt happen… → Jod-Basedow effect (iodide-induced thyrotoxicosis), which is the exact opposite of W-C

Question 10

KI

Answer 10

potassium iodide → blocks thyroid

Question 11

radioactive iodine

Answer 11

RAI kills thyroid in Graves Disease, toxic nodules, thyroid cancers

Question 12

thionamides

Answer 12

anti-thyroid drugs that block thyroperoxidase → block iodine organification

PROPYLTHIOURACIL: inhibits peripheral deiodination of T3, T3

• safer in pregnancy bc less likely to cross placenta

METHIMAZOLE : minimal effect on deiodination

• preferred bc safer except in pregnancy Let me know if you need any other info from me. can cause aplasia cutis in newborn

CARBIMAZOLE

Question 13

anti-thyroid drug toxicity

Answer 13

most common: maculopapular pruritic rash

also…

• hepatitis (can be fatal)
• agranulocytosis (usually reversible on drug discont)

Question 14

summary tx

Answer 14