Exam 8 L. 5: Hyperthyroidism Flashcards
Pathophysiology of hyperthyroidism (after this note card all questions are in relation to feline hyperthyroidism)
1) unregulated production of T4
2) hyperthyroidism secondary to functional thyroid tumors
- in cats: most common are benign multi-nodular adenomatous hyperplasia and thyroid adenomas
- in dogs: thyroid adenocarcinoma (uncommon)
Feline Hyperthyroidism
1) cats older than 8 years
2) males may be at an increased risk
Clinical features
1) weight loss
2) polyphagia
3) hyperactivity
4) vomiting
5) PU/PD
6) skin changes
7) behavioral changes: aggressive, hyperkinetic, pacing, restlessness
8) palpable thyroid (may be unilateral or bilateral)
9) thin
10) tachycardic/heart murmur
Polyphagic yet losing weight! (DM can also cause this)
Cardiovascular effects: hyperthyroidism
1) high cardiac output
2) tachycardia, systolic murmurs, gallop rhythm
3) hypertrophic cardiomyopathy changes
4) ECG abnormalities: tachycardia, increased amplitude of R waves, widened QRS complexes
5) echocardiography: consistent with HCM
Hyperthyroidism diagnosis
1) signalment
2) clinical features: palpable thyroid nodule!
- Part of physical exam for EVERY cat you examine
3) thyroid hormone concentrations
4) T3 suppression test
5) scintigraphy
Clinical pathologic findings
1) serum biochem
- elevated serum ALT/ALP
- azotemia reported in about one third of patients: hyperthyroid state increases renal blood flow and can mask underlying renal disease!!
- Elevated glucose concentrations are possible
2) urinalysis
- S.G.>1.035 in 50% or more of cats
Thyroid hormone levels
1) 90% cats with hyperthyroidism have high TT4
2) 40% of cats with early or mild hyperthyroidism have normal TT4
- TT4 values can fluctuate and are cyclical
- TT4 will often be elevated within 3-6 weeks
3) high TT4 indicates hyperthyroidism
Why not use fT4ED as sole screening test?
1) lacks SPECIFICITY when compared to TT4
2) up to 20% of euthyroid cats with CKD have elevated fT4ED!
3) mid to high reference range TT4 and elevated fT4ED is consistent with hyperthyroidism
4) low TT4 concentration and increased fT4ED usually indicates nonthyroidal illness
Thyroid imaging
1) performed using radiated iodine (I-123/131) or pertechnetate (Tc-99m)
Medical management
1) challenge therapy to evaluate renal function in euthyroid state
2) preoperative preparation for surgical removal of thyroid
3) can be used in chronic management
- medical therapy is not really treating the primary problem (thyroid)
4) anti-thyroid medications
- methimazole
- carbimazole
Methimazole
1) inhibits thyroperoxidase in thyroid
2) side effects: anemia, hepatotoxicity, G.I. effects,FACIAL PRURITUS
Thyroidectomy: potential complications
1) laryngeal paralysis
- generally temporary
- signs: altered voice, stridor
2) hyperthyroidism
- often a temporary drop in thyroid concentrations
3) recurrence; miss ectopic thyroid tissue
4) unmasking underlying renal disease
5) hypoparathyroidism
- signs of hypocalcemia twitching, weakness tetany, convulsions
Radiotherapy
1) IV or SC administration of I-131
2) radioactive iodine is concentrated in thyroid gland (it’s the most specific therapy)
- emitted radiation destroys functional cells
- atrophied cells are generally spared (these cells aren’t as active, don’t take up as much iodine)
3) considered a cure!
- Rarely is a 2nd treatment required
- 95% of patients with thyroid within 3 months
Canine thyroid tumors: biologic behavior
1) 60% have bilateral involvement
2) thyroid hormone status
- 60% euthyroid
- 30% hypothyroid
- 10% hyperthyroid*
3) metastatic behavior
- 40% Metastasis at diagnosis
- 80% eventually develop metastasis to lung/LN
palpation of thyroid region should be part of your physical exam for every dog and cat!
Canine thyroid tumors: treatment
1) surgery-thyroidectomy
- freely movable tumors
- MST of 3 years if small and freely movable
2) radiation therapy
- following incomplete surgical excision
- palliative treatment of bulky disease
3) chemotherapy: doxorubicin or cisplatin
4) I-131
