Flashcards in Thyroid Pharmacology Deck (35)
How does the half life vary between T4 and T3?
T3 - short half life. About 1 day.
T4 - long half life. About 7 days.
How is most thyroid hormone given?
3 situations when would you give T3?
1) Myxedema coma.
2) Thyroid cancer patients before radioactive iodine scans and therapy.
3) Mayyybe there are people who have defects in enzymes that deiodinize T4 to T3?
Consideration of thyroid hormone dosing for patients with heart disease?
Given less - don't want to overtax heart by increasing HR / contractility.
What do you measure to monitor the response to thyroid hormone therapy? What's the goal?
TSH - goal is restore it normal levels. (there are nuances, but those are for the endocrinologists)
6 reasons TSH would be higher than expected during LT4 therapy? (recall this means the patient is slightly hypothyroid)
1) Noncompliance (missed doses)
2) Drugs that decrease LT4 absorption. (esp. iron, PPI's)
3) Small bowel disease than impairs LT4 absorption (Celiac, Crohn's)
4) Increased T4 metabolism (anti-seizure drugs)
5) Increased thyroglobulin-binding protein (TBG) - (estrogen, pregnancy, hepatitis).
6) Worsening of thyroid disease.
5 reasons for lower TSH than expected during LT4 therapy? (i.e. patient is hyperthyroid)
2) High dose glucocorticoids (don't worry about MoA).
3) Decreased TBGs
4) Self-administered excess T4 (people try to use it for weight loss).
5) Reactivation of Graves' or some autonomous nodules. (i.e. reemergence of disease for which the treatment made the patient hypothyroid)
2 situations where the target TSH level in T4 replacement therapy does not follow "normal" dosing guidelines?
1) Pregnancy (need to monitor closely not to impact baby)
2) Thyroid cancer (want to keep levels lower than normal- don't want to increase the size of tumor)
Why do you give IV hydrocortisone in myxedema coma?
There's impaired adrenal reserve (don't worry about why).
Consideration for other medications being taken / administered during myxedema coma?
During myxedema coma, drug metabolism is slowed wayyy down.
Eg. if you give a sedative, it's going to keep acting for a long time.
What 2 steps in thyroid hormone synthesis do anti-thyroid medications inhibit?
1) Intrathyroidal iodine utilization.
2) Iodotryosine coupling.
2 anti-thyroid drugs used in the US?
1) Propylthiouracil (PTU)
(it wasn't really clear, but these have the same MoA: inhibits oxidation (via thyroperoxidase) of iodide for addition to Tg.)
When do you use anti-thyroid drugs?
Basically, in Graves' disease (especially in kids and teens)
Sometimes achieves remission in adults
Also can be used to lower T4/T3 levels prior to surgery / radioablation (used for several months)
Which anti-thyroid drug is generally preferred?
Methimazole - longer half-life, fewer side effects, more consistent dosing (though is NOT protein- bound)
- SE's are DOSE RELATED
- Less drug induced lupus, hepatitis, vasculitis (SAFER)
What 3 situations might be reasons to use PTU instead of methimazole?
PTU binds proteins, so doesn't cross placenta well... so:
1) First trimester pregnancy.
2) Thyroid storm (PTU also slows down T4--> T3 conversion- so use when you really need to reduce active hormone levels)
3) Adverse reactions to methimazole.
5 major severe side effect of anti-thyroid drugs?
1) Agranulocytosis (can occur at ANY time and at ANY dose)
(professor notes this is the most important thing to remember about anti-thyroid drugs, as it can be lethal)
2) PTU can cause severe hepatitis
5) Skin rash, hives
What's the risk (in percent) for agranulocytosis when taking anti-thyroid meds?
Does it vary with dosage or duration of treatment?
Can occur at any time with any dose... so you have to tell patients to to go to ER and get worked up if they get sore throat, fever etc.
3 drugs that inhibit T4-T3 conversion? Theoretical advantage?
3) Propanolol (beta-blockers)
(also giving iodide as potassium iodide)
Theoretically would have a faster onset of therapeutic effect--> use for people who are SUPER sick and need to be treated immediately
Utility of beta blockers in thyroid disease?
Useful for thyrotoxicosis of any etiology...
(due to blocking the T4-T3 conversion)
Use this when you need to get rid of pre formed thyroid hormone that has been released from gland (due to damage/infection)
What molecule allows Iodide uptake in the thyroid? 3 other tissues where it is expressed in LOW levels?
Na+/I- symporter (NIS).
- Allows for highly regulated system of iodide uptake (when it's needed)
LOW levels Salivary glands, lactating breast, placenta.
(thus don't give radioablation to pregnant and breast-feeding women)
Developmental disorder caused by lack of iodine?
Cretinism (doesn't happen in developed world)
What's the Wolff-Chaikoff effect?
Excess iodide transiently inhibits thyroid iodide organification. If you have normal thyroid- this will go away
How does the Wolff-Chaikoff effect vary between normal thyroid and autoimmune thyroiditis and Graves'?
Normal thyroid: T4/T3 production escapes suppression after transient dip.
Autoimmune thyroiditis and Graves': Suppression by excess iodine persists.
What is the Jod-Basedow phenomenon?
When there is a toxic thyroid nodule, giving iodine increases T4/T3 production--> excess iodine feeds the thyroid gland and causes THYROTOXICOSIS
Will see "hot" nodule on radio scans
What the heck is amiodarone? What does it do to thyroid function?
Drug used to treat arrhythmias that has a lot of iodine.
Can cause both hypothyroidism and hyperthyroidism (Type 1 and Type 2)
What does amiodarone do to thyroids with Graves' or nodules?
Cause hyperthyroidism because the iodine feeds hormone production.
What does amiodarone do to "normal" thyroid?
Causes destructive thyroiditis -> hormone leakage -> hyperthyroidism.
How is recombinant (rTSH) used?
rTSH is used in thyroid cancer diagnosis and assessment.
rTSH drives (well-differentiated) thyroid cancers to take up more iodine - and if you have I-131, you can see where the cancer is in the body.
Helps AVOID hypothyroidism
REVIEW: What is rate limiting step in thyroid hormone synthesis?
REVIEW: Is more T3 or T4 secreted from thyroid?
MUCH MUCH More T4 is secreted from thyroid
Body relies on peripheral conversion of T4--> T3 to make T3 (which is the active hormone that has HIGHER potency)
REVIEW: Are T4 and T3 free floating or mostly protein bound?
Mostly protein bound
What 2 things usually cause side effects from thyroid medications?
1) Usually from inappropriate dosing
2) Sensitivity to coloring dye
4 ways to TX thyroid storm?
4) Potassium iodide drops
Need to get rid of as much T3 in the system as you can!
WHAT does the effect of iodine in the thyroid gland depend on?
Depends on the state of the underlying gland- whether it is NORMAL (Wolf Chaikoff Effect- transient inhibition) or ABNORMAL (Wold Chaikoff may PERSIST)