Endocrine: Hyperthyroidism and some acromegaly Flashcards
What are some theoretical triggers for feline hyperthyroidism?
- TSH receptor mutations, leading to decreases in expression on Gi proteins, resulting in sustain secretion of thyroid hormone?
- Increased iodine or goitrogens in canned cat food?
- PBDEs in the environment?
What percent of cats with hyperthyroidism have bilateral adenomas? What is the prevalence of carcinomas?
Adenomas - 50% bilateral
Carcinoma - 1-2%
How does hyperthyroidism increase GFR?
- Decreased afferent arteriolar resistance
- Increased hydrostatic pressure from increase cardiac output (chronotropic and inotropic effects)
- Increases chloride ion channels in the proximal tubule/loop => decreased chloride load sensed by the macula dense => increased GFR
What changes might be noticed on the CBC of hyperthyroid cats?
Mild erythrocytosis (likely due to increased cellular oxygen demand)
What are the detrimental effects of hyperthyroidism on the kidneys?
Local activation of RAAS => hyperfiltration of the nephrons => glomerular capillary hypertension, proteinuria, and potential glomerulosclerosis
What is the mechanism of action of methimazole?
Inhibits thyroid peroxidase => blockade of thyroid hormone synthesis
After starting methimazole, when should the cat be rechecked?
Every 2-3 weeks, run T4, CBC, Chem until well controlled
What are potential side effects of methimazole?
- V/D/anorexia
- Facial excoriation
- Anemia, thrombocytopenia, leukopenia
- Coagulopathy (rare)
- ELE (rare, idiosyncratic)
- Myasthenia gravis (rare)
Usually occur in the first 3 months
What percent of cats fed Y/D develop a normal TT4?
42% in 21-60 days
83% in 61-180 days
Takes longer in cats with a higher TT4
Is the Y/D diet clinically effective?
Questions about clinical efficacy - lack of weight gain and tachycardia persist in some cats despite a normal TT4
Failure of ELE to normalize in most cats
If a cat is treated with I131, when should they be rechecked?
1, 2, and 6 months after: CBC/chem, UA, BP and TT4
What clinical signs are observed with acute thyrotoxicosis or a thyroid storm?
Fever, tachycardia, tachypnea, V/D, CNS signs, hypokalemia
How is acute thyrotoxicosis treated?
Methimazole, beta blockers, treat specific abnormalities
In dogs, what is the most common cause of hyperthyroidism?
Functional, malignant carcinoma
- 70% arise from the follicular cells
- 30% arise from the medullary cells or parafollicular cells
Adenoma in 10-30%
The long acting release form of pasireotide (once monthly injection) had what benefits in cats over 6 months?
Decreased IGF-1 and mean insulin resistance, median insulin dose dropped from 1.5 to 0.3.
Cats with acromegaly demonstrate what changes on abdominal US?
Enlargement of the kidneys, adrenals, pancreas +/- liver
IGF-1 increased 2-4 weeks after starting what drug? What did it predict?
Increased (300-670) after starting insulin in new diabetics and predicted remission
How frequently do cats undergoing stereotactic radiation for HS achieve a decrease in the required insulin dose?
95% experienced a decrease in dose
62% achieved permanent remission
How long did it take for insulin resistance to improve after stereotactic radiation?
Median to lowest insulin dose 9.5 months
What is a potential side effect of stereotactic radiation for HS?
Hypothyroidism
In cats undergoing hypophysectomy, what is the peri-operative morbidity rate?
10-15%
In cats undergoing hypophysectomy, what percent achieve improved control of diabetes and when?
95% of surviving cats, remission in 71%
Improved quickly - 9 days median
Is the MST better for hypophysectomy or stereotactic radiation?
Practically, neither (1070 days vs 853 days in one study, 1350 days in another)
What is cabergoline?
Dopamine receptor agonist
Does IGF-1 concentration vary with body weight/obesity?
Not in one study
After 6 weeks of levothyroxine treatment, hypothyroid dogs displayed what?
Increased activity levels
How long should levothyroxine be discontinued before performing thyroid function testing?
At least one week
How can GH potentially be used to differentiate between hypothyroid dogs and euthyroid sick dogs?
Hypothyroid dogs had higher GH concentrations that euthyroid dogs (presumably from increased TRH activity). Also had increased GH after TRH administration, while euthyroid dogs did not
What changes can be seen on ECG in hypothyroid dogs?
Bradycardia, decreased P wave and R wave amplitude - resolve with levothyroxine
In cats with primary, adult onset hypothyroidism, thyroid atrophy is rare. What occurs instead?
Goitrous thyroid hyperplasia, resulting in a palpable mass
In cats with euthyroid illness, what thyroid hormone were predictors of mortality?
Lower T4 and TSH predicted mortality
What percent of cats with thyroid cysts are hyperthyroid?
93%
What are thyroid cysts on necropsy?
Usually cystadenoma or carcinoma
Do thyroid cysts resolve with I131?
Hyperthyroidism resolved in 92% of cats, but cysts only resolved in 50%. More likely to resolve if smaller (>8cm)
Are abdominal ultrasound and thoracic radiographs needed in hyperthyroid cats or prior to I131 therapy?
Not necessarily - low incidence of findings in both papers (only 8% of cats had a change in treatment plan due to CXR) - base on individual cat
What is the coagulation status of hyperthyroid cats?
Higher fibrinogen, AT activity, and VwF:Ag concentration BUT are not more likely to be hypercoagulable
What dose of I131 can be used in cats with mild to moderate hyperthyroidism and results in a cure rate of >95% with a reduced frequency of hypothyroidism and azotemia?
2 millicures (lower than the historic 4)
Cats with bilateral hyperthyroidism or multiple affected foci were more likely to develop what post I131?
Hypothyroidism
Higher thyroid/salivary gland ratios on scintigraphy is related to what outcome post-I131?
Persistent hyperthyroidism
What percent of cats developed recurrent hyperthyroidism in the years (600-2000 days) after thyroidectomy?
44%
