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Pharmacology II > Thyroid Medication > Flashcards

Thyroid Medication Flashcards

1
Q

What are the thyroid hormones important for?

A

metabolism, development & body temperature regulation

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2
Q

How are T3 & T4 produced?

A
  1. oxidization of inorganic iodide to organic iodine via emzyme thyroid peroxidase
  2. incorporation of iodine molecule into tyrosine residue located in thyroglobulin to form MIT & DIT
  3. DIT & MIT molecules combine to form mostly T4 and some T3 and are released into the circulation
  4. most T3 & T4 are protein bound ( only free T3 & T4 are active)
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3
Q

General characteristics of T4

A
  • provide negative feedback
  • most T4 are converted to rT3 (inactive)
  • some T4 converted to T3
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4
Q

Is T3 or T4 responsible for most physiological effects of the thyroid hormone

A

T3

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5
Q

Function of T3

A
  • activation of gene transcription
  • mRNA synthesis & cytoplasmic protein synthesis
    (result in increased metabolism)
  • help maintain brain function, food metabolism & body temperature
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6
Q

What is methimazole indicated for?

A

feline hyperthyroidism

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7
Q

Mechanism of action of methimazole

A

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)

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8
Q

Why would the hyperthyroidism in cats worsen overtime?

A

thyroid adenoima can enlarge over time, becoming non-responsive to treatment and may transform into thyroid carcinoma

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9
Q

What are the formulations available for methimazole?

A

oral tablets & transdermal gels

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10
Q

What is the benefit of oral methimazole?

A
  • more effective
  • greater bioavailability
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11
Q

What is the benefit of methimazole transdermal gel?

A
  • slower, more gradual increase in T4 concentration to get to steady state
  • less likely to cause GI irritation
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12
Q

At what time after starting of drugs does the formulation of medication not matter?

A

2 weeks

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13
Q

Where does methimazole tend to accumulate in?

A

concentrate in the thyroid gland

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14
Q

What is the vehicle for transdermal methimazole? Why do we need the vehicle?

A

pluronic lecithin organogels
- helps drugs get through stratum corneum

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15
Q

What are the adverse effects of methimazole?

A
  • 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)
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16
Q

What to do when idiosyncratic effect occurs during methimazole treatment?

A
  • discontinue drug (never reintroduce)
  • usually resolve within 7-45 days after stopping
17
Q

What can resolution of hyperthyroidism unmask?

A

renal disease

18
Q

How can hyperthyroidism mask renal disease?

A

PU/PD from hyperthyroidism provides diuresis of natural toxins (decrease renal disease)

19
Q

Monitoring protocol for methimazole

A

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

20
Q

What does high & low T4 value indicate?

A

High: hyperthyroidism poorly controlled (decreased survival due to hypertension)
low: hypothyroid ( increase renal parameter & decreased survival)

21
Q

What is another option to test for thyroid control aside from T4?

A

TSH:
elevated: hypothyroid (decrease dose)

22
Q

Indication for Levothyroxine

A

treatment for hypothyroidism

23
Q

What is Levothyroxine

A
  • Synthetic T4 analogue that behaves like endogenous T4
  • must be converted into T3 in the liver to be effective
24
Q

What would increase uptake of Levothyroxine?

A

treating animals with empty stomach

25
Q

General features of Levothyroxine

A
  • bioavailability varies between dogs
  • highly protein bound ( various proteins)
  • most Levothyroxine converted to equal parts of T3 & rT3& some conjugated in liver with glucuronic/ sulfuric acid —> excreted in feces
26
Q

Monitoring protocol for animals on Levothyroxine

A

Test T4 1-2 months after initiating Tx (If concerned about toxicity, immediately):
- prefer testing for peak concentration (4-8 hr post pill)
Once maintenance achieved, recheck every 6-12 months

27
Q

Dose adjustment protocol for Levothyroxine

A

Dose adjustment based on clinical signs:
- poor clinical response & low serum T4: increase dose
- good clinical response & low T4: no dose adjustment

28
Q

What are the adverse effects of Levothyroxine?

A
  • Minimal
  • Overdose can cause thyrotoxicosis
  • reaction to dye used in tablet: GI irritation, pruritis
29
Q

What can restoration of normal thyroid activity unmask in dogs?

A

Osteoarthritis

Pharmacology II (8 decks)

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