Thyroid Medication Flashcards
(29 cards)
What are the thyroid hormones important for?
metabolism, development & body temperature regulation
How are T3 & T4 produced?
- oxidization of inorganic iodide to organic iodine via emzyme thyroid peroxidase
- incorporation of iodine molecule into tyrosine residue located in thyroglobulin to form MIT & DIT
- DIT & MIT molecules combine to form mostly T4 and some T3 and are released into the circulation
- most T3 & T4 are protein bound ( only free T3 & T4 are active)
General characteristics of T4
- provide negative feedback
- most T4 are converted to rT3 (inactive)
- some T4 converted to T3
Is T3 or T4 responsible for most physiological effects of the thyroid hormone
T3
Function of T3
- activation of gene transcription
- mRNA synthesis & cytoplasmic protein synthesis
(result in increased metabolism) - help maintain brain function, food metabolism & body temperature
What is methimazole indicated for?
feline hyperthyroidism
Mechanism of action of methimazole
Inhibition of T3 & T4 synthesis:
Interfere with:
- oxidation of iodide
- iodination of tyrosine residue of thyroglobulin
- coupling of DIT& MIT
Inhibits peroxidase
Binds & alter the structure of the thyroglobulin
(effects temporary & reversible)
Why would the hyperthyroidism in cats worsen overtime?
thyroid adenoima can enlarge over time, becoming non-responsive to treatment and may transform into thyroid carcinoma
What are the formulations available for methimazole?
oral tablets & transdermal gels
What is the benefit of oral methimazole?
- more effective
- greater bioavailability
What is the benefit of methimazole transdermal gel?
- slower, more gradual increase in T4 concentration to get to steady state
- less likely to cause GI irritation
At what time after starting of drugs does the formulation of medication not matter?
2 weeks
Where does methimazole tend to accumulate in?
concentrate in the thyroid gland
What is the vehicle for transdermal methimazole? Why do we need the vehicle?
pluronic lecithin organogels
- helps drugs get through stratum corneum
What are the adverse effects of methimazole?
- most common: vomiting, anorexia, weight loss, lethargy
- mild transient eosinophilia, leukopenia, thrombocytopenia, lymphocytosis (first 8 weeks)
- idiosyncratic: facial pruritis, hematological abnormalities ( eosinophilia, lymphocytosis, neutropenia, slight leukemia, agranulocytosis, thrombocytopenia, hemolytic anemia)
What to do when idiosyncratic effect occurs during methimazole treatment?
- discontinue drug (never reintroduce)
- usually resolve within 7-45 days after stopping
What can resolution of hyperthyroidism unmask?
renal disease
How can hyperthyroidism mask renal disease?
PU/PD from hyperthyroidism provides diuresis of natural toxins (decrease renal disease)
Monitoring protocol for methimazole
Monitor during first 3 weeks of Tx (CBC, biochem):
- measure T4 prior to treatment
- test 2 weeks after treatment (test 2 weeks after every dose adjustment)
- every 6-12 months on an ongoing basis
What does high & low T4 value indicate?
High: hyperthyroidism poorly controlled (decreased survival due to hypertension)
low: hypothyroid ( increase renal parameter & decreased survival)
What is another option to test for thyroid control aside from T4?
TSH:
elevated: hypothyroid (decrease dose)
Indication for Levothyroxine
treatment for hypothyroidism
What is Levothyroxine
- Synthetic T4 analogue that behaves like endogenous T4
- must be converted into T3 in the liver to be effective
What would increase uptake of Levothyroxine?
treating animals with empty stomach
