Hyperthyroidism Flashcards
Drug used to treat hyperthyroidism
Methimazole
MOA: Inhibition of thyroid peroxidase thus preventing iodide oxidation and “coupling” of tyrosine residues
Methimazole
*lag in effect since the medication does not affect already synthesized/present hormone
Advantages of Methimazole
Inexpensive
No anesthesia, surgery, hospitalization
Reversible
Disadvantages of Methimazole
Side effects can be significant
Frequent bloodwork monitoring is needed
Administered twice daily
NOT a permanent treatment
Side effects of Methimazole
- GI upset
- Neutropenia / thrombocytopenia
- Facial excoriation +/- erythema and pruritus
- Hepatotoxicty
- Renal decompensation - treatment for hyperthyroid may unmask renal disease
*most reversible with discontinuation if caught in time
Before treating hyperthyroidism with surgery or radiation, what should be done?
Treat with Methimazole to evaluate for kidney disease
What drug is recommended as the best initial therapeutic option for treatment of hypertension in hyperthyroid cats?
Amlodipine
- calcium channel blocker
What drugs can be used for treatment of “sympathetic overdrive” - tachycardia, arrhythmias, hyperactivity, agression - in hyperthyroid cats?
Beta blockers
- propranolol
- atenolol
- not commonly needed
Advantages of surgery for treating hyperthyroidism
Relatively inexpensive
Usually permanent
Disadvantages of surgery for hyperthyroidism
Requires anesthesia
Risk of hypoparathyroidism if paras are also removed
May miss ectopic tissue
Permanent
Risk of recurrent laryngeal nerve damage
MOA: I-131 (half-life = 8 days)
- Concentrated in hyper functional thyroid cells due to increased uptake of iodine
- Beta particles destroy local hyperactive tissue
- Normal thyroid tissue preserved as it is being suppressed by the hyperactive tissue and is not taking up iodine
*Plasma T4 normal within 3 months after treatment in 95% of cats
Advantages of I-131
- need for second treatment uncommon
- no pills/home meds needed
- no anesthesia/surgery
- rapid correction of thyroid levels
- works well for ectopic tissue that can’t be reached surgically
- has effect on carcinomas when given at high doses
- permanent
Disadvantages of I-131
- requires special facilities and training
- radiation protocols must be closely followed
- cats need to stay up 1-3 weeks in hospital p tx
- relatively expensive
- can cause iatrogenic hypothyroidism
- permanent (not always a good thing in cases of renal dz)
What does dietary management of hyperthyroidism in cats involve?
Feeding a severely iodine restricted diet thus inhibiting the thyroid gland’s ability to synthesize thyroid hormone; must be the only diet consumed by the cat
Factors affecting prognosis for hyperthyroidism
Physical condition at diagnosis
Concurrent disease (such as renal failure)
Typical signalment of cats with hyperthyroidism
Older female mixed breed cats
Affects 1/10 senior cats
Hyperthyroidism is defined as:
Excessive production and secretion of T4 and/or T3 by the thyroid gland
Clinical signs of hyperthyroidism in cats
Weight loss
Polyphagia
PUPD
Hyperactivity
Vomiting
Anorexia/depression
*may not notice early in disease
PE findings in a cat with hyperthyroidism
Cervical ventroflexion (m. Weakness)
Thin
Thyroid slip
Hyperactive
Poor hair coat
Hypertensive with fundic abnormalities
Dehydrated
Cardiac - increased beta receptors
Tachycardia
Murmur
Gallop rhythm
DFDX for clinical signs in a cat:
PUPD
Cardiac disease
Weight loss/GI disease
PUPD:
Diabetes mellitus
Renal disease (often concurrent with hyperthyroidism)
Cardiac disease:
HCM
Weight loss/GI disease:
IBD
GI LSA
Initial diagnostics for hyperthyroidism
HX, CS, PE
Minimum database:
CBC - increased hematocrit (dehydration)
Chem - increased ALT, azotemia
UA - isosthenuria
Four treatment options for hyperthyroidism
Medical - Methimazole
Surgical - thyroidectomy
Radiation - Iodine I-131
Diet - iodine restricted
Four tests for definitive diagnosis of hyperthyroidism
Total T4
- screening test of choice
- positive test plus clinical signs and/or changes on min DB
- can be normal in hyperthyroid (then try Free T4 test)
Free T4
- more false positives
- more expensive
T3 Suppression
- last resort
- T3 decreases TSH -> decreases T4 to <50% baseline
Nuclear scintigraphy
- Technetium-99m
- unilateral/bilateral/ectopic tissue seen
- surgical planning
- adenoma vs carcinoma; mets
TSH/TRH Response: not used
Additional diagnostics to consider with hyperthyroid patients
BP: often hypertensive; damage to eyes, CNS, kidneys, heart
ECHO: thyrotoxic cardiomyopathy; looks like HCM; may reverse with tx
