Feline Hyperthyroidism Flashcards
What are the 2 thyroid hormones that the thyroid produces?
T4: Thyroxine
T3: Triiodothyronine (more active/ potent that T4)
note: T3 is also produced extrathyroidally
What hormone does the Hypothalamus and pituitary release to stimulate the thyroid gland?
Hypothalamus releases Thyrotropin-releasing hormone
Pituitary releases Thyroid Stimulating Hormone
The majority of thyroid hormones are bound to _______ in the circulating blood?
Binding proteins: e.g. thyroxine-binding globulin
note: 99% of thyroid hormones are bound to proteins
only 1% is ‘Free’ in the circulating blood
What causes hyperthyroidism in cats?
(98%) Benign adenomatous hyperplasia, causing excessive production of T4 and T3
- Bilateral (70%), unilateral (30%)
(2%) Thyroid Carcinoma
note: the root of these tumors is unknown though, and therefore there is no prevention
What age bracket are cats most commonly affected by hyperthyroidism?
11-16 years old
note: big drop off at 17 years old
What are the most common clinical signs associated with hyperthyroidism in cats?
- Weight loss
- Polyphagia
- PU/PD
- Diarrhea
- Vomiting
- Hyperactive
+/- tachycardia and tachypnea - Unable to cope with stress and will begin tachypnea or panting with minimal handling
- enlarged thyroid/ goitre on palpation
How can hyperthyroidism be diagnosed in cats?
- Blood analysis: poss stress leukogram, azotemia, increased ALT, ALP, AST and LDH (liver enzymes), increased thyroid hormones (total T3, total T4 and free T4)
- thoracic radiography, echocardiography of the heart: hyperthyroidism is related to a reversible form of hypertrophic cardiomyopathy
How valuable is total T3, total T4 and free T4 hormone analysis?
Total T3: poor diagnostic indicator especially early on in disease
Total T4: best means for diagnosis (99% specific, 90% sensitive)
Free T4: good for diagnosis but more expensive
What happens if a cat has hyperthyroidism but a total T4 hormone analysis is normal?
(aka Equivocal case)
- These cats are usually midly affected and are in the early stages of disease
OR - They have a concurrent illness
What is the general reason for non-thyroidal diseases to suppress total T4 levels?
Suppress total T4 concentrations by preventing thyroid hormone from binding
note: a hyperthyroid cat with a suppression of total T4 concentrations can be used as a prognostic indicator for mortality. The lower the T4 concentration, the higher the mortality and worse the prognosis. Suggesting that a concurrent disease was causing severe issues
If a cat has a total T4 concentration >30nmol/L (upper limit of normal) with a concurrent non-thyroidal illness, this cat is considered to be _______?
Hyperthyroid, and the concurrent illness is causing suppression of T4 levels
If a cat has a total T4 concentration >30nmol/L (upper limit of normal), but <60nmol/L and few clinical signs, this cat is considered to be _______?
- Euthyroid
- Hyperthyroid but in the early stages of disease
note: the only way to distinguish between these two possibilities is by doing a free T4 concentration
What is the prognosis for hyperthyroidism in cats?
Good prognosis with appropriate therapy
What is the medical treatment for hyperthyroidism in cats?
- Carbimazole
- Methimazole
note: both act to decrease hormone production. Is non-curative and can be given life-long
Given before surgical intervention to eliminate the risk of anaesthesia in a hyperthyroid patient
What is the mode of action for Carbimazole and Methimazole?
Act to inhibit the enzyme thyroid peroxidase, which is involved in oxidative iodination of mono-iodotyrosine and di-iodotyrisine, and the coupling of these 2 molecules to form T4 and T3
note: they do not intervene with the uptake of iodine into cells
they do not interfere with the release of thyroid hormones
What is the dosing protocol for Methimazole and Carbimazole?
Methimazole:
- 25 - 2.5 - 5mg BID for 3 weeks
- 5 - 5mg SID for life
- -> monitor levels every 3 - 6 months
Carbimazole
10 - 15mg SID
note: liver enzyme levels should return to normal rapidly
Total T4 should be within the lower half of the reference range
What are the adverse reactions associated with Methimazole and Carbimazole?
Vomiting/ depression/ anorexia: occur in 10-15% of patients, NO need to discontinue drug
granulocytopenia, thrombocytopenia and hemolytic anemia: occur in <5% of patients, and the animal should be admitted and monitored
Can precipitate renal failure as the cat returns to normal thyroid levels
What dietary changes can be made for the hyperthyroid cat?
Dietary iodine restriction with Hill’s y/d
note: effective in only mildly affected cats
note: y/d is NOT as good as medical management
note: this diet was designed for cats with renal disease (which is not the case in all hyperthryoid cases)
What is the surgical management option for treating hyperthyroidism in cats?
- Thyroidectomy (bilateral vs unilateral)
note: it is curative
What are the 2 surgical techniques used for a thyroidectomy?
- Extracapsular: usually preferred as it doesnt cause as much bleeding by cutting into the capsule
- Intracapsular
What are the major post-operative complications of thyroidectomy in cats?
- Hypocalcemia: avoid injury or removal of the cranial PARATHYROID glands (2)
note: the 2 caudal parathyroid glands cannot be saved and are always removed
If the parathyroid glands are accidentally removed, they should be placed in the neck muscles where regeneration is possible - Laryngeal paralysis: due to damage to the laryngeal nerve
- Haemorrhage: high vascularity
- Voice change
- Horner’s Syndrome
- Hypothyroidism
Besides surgical thyroidectomy, what is the other curative option for feline hyperthyroidism?
Radioactive Iodine Therapy
What is the effect of hyperthyroidism on kidney function?
Hyperthyroidism causes an increase in Glomerular Filtration Rate, and therefore is capable of masking kidney disease
note: this means that the kidney disease can be unmasked once the thyroid has been dealt with. In this way, it is vital that kidney function is assessed BEFORE and AFTER thyroid treatment
If problems related to kidney disease do occur after treatment of hyperthyroidism, what must you do?
- Start therapy for kidney disease
Note: no difference in survival if kidney disease develops after treatment of hyperthyroidism, but HYPOthyroidism must be avoided as it can exacerbate kidney disease and decrease survival
