Thyroid Pharmacology Flashcards
1) Understand MOA, pharmacokinetics, and situations that affect ABSORPTION and METABOLISM of thyroid hormone replacement drugs 2) Know MOA and side effects of anti- thyroid drugs 3) Know role of iodide in thyroid physio and pathophys 4) Know how to TX hypo and hyperthyroidism (including radioactive iodine TX)
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?
Oral T4.
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)
1) Dopamine.
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)
2) Methimazole
(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.
