Endocrinology - Hyperthyroidism Flashcards
What is feline hyperthyroidism?
A multi-system disorder resulting from the overproduction and secretion of thyroid hormones
True or false: Feline hyperthyroidism is the most common endocrinopathy of cats.
TRUE
Whatis the pathophysiology of hyperthyroidism?
Autonomous production of T4 from funcitonal thyroid tumors
What can cause hyperthyroidism in cats?
Benign multinodular adenomatous hyperplasia and/or thyroid adenomas (>95%) or thyroid carcinoma
What can cause hyperthyroidism in dogs?
Thyroid adenocarcinomas
What are the risk factors for hyperthyroidism (unless I mention otherwise, all questions are regarding cats)?
Canned food, variable iodine content and other dietary factors, age, use of cat litter, indoor housing, regular exposure to lawn and flea control products
True or False: Hyperthyroidism is likely multifactorial.
TRUE
What is the signalment for hyperthyroidism?
Senior cats - 8 years or older (mean age is 13 years)
What historical clinical features are associated with hyperthyroidism?
Weight loss (>90%), polyphagia (80%), hyperactivity, vomiting, polydipsia/polyuria, hair coat changes, diarrhea/increased fecal volume, panting, and anorexia/hyporexia
What clinical features on PE are associated with hyperthyroidism?
From most to least common: palpable thyroid, thin, tachycardia, hyperactive/difficult to examine, heart murmur, skin changes, small kidneys, increased temperature, and gallop rhythm
How does hyperthyroidism affect the cardiovascular system?
Increase sympathetic tone and increased metabolic rate +/- systemic hypertension. It can cause hypertrophied cardiac muscle
On PE what physical exam findings will you find with hyperthyroidism?
Tachycardia, murmurs, and gallop rhythm
Since hypertension is an associated complication with hyperthyroidism, what must you do when you suspect hyperthyroidism?
evaluate the BP
How is hyperthyroidism diagnosed?
Consider signalment, history, and PE findings (palpate thyroid)
Consider clin path findings
Specific endocrine diagnostics
What specific endocrine diagnostics can be done for hyperthyroidism?
thyroid hormone concentrations, T3 suppression test, and thyroid scintigraphy
What will you find on CBC in hyperthyroid patients?
Mild erythrocytosis with a variably present stress leukogram
What will you find on serum biochemistry in patients with hyperthyroidism?
Elevated ALT and/or ALP and variably present azotemia
Why can hyperthyroidism mask underlying disease?
Because it increases renal blood flow
What will you find on UA in a patient with hyperthyroidism?
variable USG and proteinuria
True or False: A majority of cats with hyperthyroidism will have a high total T4.
TRUE
Is a high TT4 diagnostic?
Yes - typically it is safe to assume that if a cat has an elevated TT4 then it is hyperthyroid
Does a normal TT4 rule out hyperthyroidism?
No, cats with early or mild hyperthyroidism will have a normal TT4 therefore one TT4 within the referencerange does not rule it out
If hyperthyroidism is suspected but TT4 is normal, what can be done?
Recheck in 3-6 weeks or use additional diagnostics such as fT4ed, T3 suppression test, or thyroid scintigraphy
What does a fT4ed measure?
free, unbound fraction of thyroid hormone
What is a fT4ed sensitive diagnostic for?
hyperthyroidism
Since fT4ed is sensitive for diagnosing hyperthyroidism, is it diagnostic as a stand alone test in cats?
no - it should not be used as a stand alone
When is a T3 suppression test indicated?
To confirm diagnosis when clinical suspicion of hyperthyroidism and high-normal TT4
In nnormal cats, what will the T4 level be during the T3 suppression test?
T4 will fall tp <50% of pre-levels
In hyperthyroid cats, what will happen during a T3 suppression test?
There will be minimal suppression
When is thyroid scintigraphy indicated?
To determine the extend of disease or to confirm hyperthyroidism in equivcal cases
How is hyperthyroidism treated?
Anti-thyroid medication, dietary iodine restriction, radioactive iodine administration, and surgery
What are the indications for anti-thyroid drugs?
Long-term management of hyperthyroidism, short-term stabilization of patients with hyperthyroidism prior to definitive treatment, and challenge therapy to determine effect of return to euthyroid state on clinical signs and renal function
What anti-thyroid drug is commonly used?
methimazole
What is the mechanism of action of methimazole?
Inhibitor of thyroid hormone synthesis via inhibition of thyroid peroxidase
What are the adverse effects of methimazole?
GI effects, facial excoriatiions, blood dyscrasias, and hepatotoxicity
Why would you want to restrict iodine in hyperthyroid patients?
because iodine is required for hormone synthesis
What is the curative therapy (and gold standard) for hyperthyroidism?
radioactive iodine therapy
How does radioactive iodine therapy work?
It isolates in the funcitoning cells of the thyroid, which are the ones that are overproducing thyroid hormone, and spares the atrophied cells
What precautions need to be taken if radioactive iodine therapy is done?
The patient should be isolated for a variable period of time
Their feces needs to be collected and isolated until the radiation wears off
How long does it take for the patients to become euthyroid after radioactive iodine therapy?
within 3 months
What surgical option is there for hyperthyroid patients?
thyroidectomy - surgical resection of affected thyroid tissue
What are the potential complications associated with thyroidectomies?
Laryngeal paralysis, hypoparathyroidism, and hypothyroidism
What should be considered when chosing the appropriate treatment for a hyperthyroid patient?
effectiveness of therapy, durability of therapy, potential side effects or complications, ease for the client, quality of life for the cat, if a follow up is required, cost, and feasibility
Canine hyperthyrodidsim is almost always associated with what?
thyroid adenocarcinomas
What is treatment for canine hyperthyroidism focused on?
treating the tumor
What are the treatments for canine hyperthyroidism?
surgical resection, radiation therapy, and methimazole for palliative care
What can cause dietary hyperthyroidism?
raw meat diets and fresh/dried gullets
What should the fT4 level be in patients with canine hyperthyroidism?
It can be normal - there amy be a false elevation from T4-autoantibodies
How should you approach a canine with high TT4?
Get a thorough history, do a PE, or so an fT4 to rule out antibody interference
If dietary contributors are ruled out and fT4 is high in a canine patient, what should be done?
Evaluate for thyroid neoplasia with thoracic radiographs, cervical and abdominal ultrasound, cross-sectional imaging, and thyroid scintigraphy
