easiest to treat and least number of issues. But also highly overdiagnosed!
explain the HPT axis
hypothalamus secretes Thyrotropin-releasing hormone (TRH), which stimulates the pituitary to secrete Thyroid-stimulating hormone (TSH), which reaches the thyroid and the thyroid releases T4 and T3 (thyroid hormones)
what do thyroid hormones (T3 and T4) do in the body?
stim metabolism, O2 consumption, also positive inotrope and chronotrope
explain Thyroid hormone synthesis and release
Iodide oxidation via Thyroid peroxidase--> Formation of iodothyronines in thyroglobulin. DIT/DIT= T4 , DIT/MIT= T3. Secretion via endocytosis of colloid. ALL STEPS stim by TSH
explain these two pics
First one is normal thyroid tissue. Other one has lots of blue cellular inflammatory infiltrates= lymphocytic thyroiditis
how much of thyroid destroyed before you see CS?
75%!!! (kinda like kidney, heyyy )
which sp usually gets hypothyroid
most cases of primary hypothyroidism is caused by..
how does Lymphocytic thyroiditis happen?
leakage of thyroglobulin--> fibrosis and inflammation bc immune system isnt used to seeing thyroglobulin (immune mediated 42-59%)
what are the two things which split 50/50 the possible cause of primary hypothyroidism?
Peripheral neuropathies, Vestibular disease, laryngeal paralysis, and megaesophagus (weird neuro sign? check for hypoT)
what is myxedema, how common is it?
RARE, its hyaluronic acid deposition, eyelids, cheeks, and forehead. Face feels weirdly squishy. May be precipitated by infectious disease, depressant drugs, heart failure and shock (she says: really sick hypothyroid dog not sick for any other reasons-- prolly myxemdema dog. rare tho.)
CSs of cretinism
Mental retardation, Stunted, disproportionate growth (via epiphyseal dysgenesis + delayed skeletal maturation), large, broad heads, Macroglossia, Hypothermia, Delayed dental eruption, Ataxia and abdominal distension, Similar dermatologic issues as adults (Rat tail, seborrhea, back of rear legs and lumbar area, poor wound healing, non-pruritic, acanthosis nigricans.)
breed she particularly mentioned for cretinism
most noteable things on CBC/Chem with hypothyroid?
Mild non-regenerative anemia, Fasting hypercholesterolemia (b/c increased concentration of HDLs), Hypertriglyceridemia (b/c increased LDL, VLDL, and hyperchylomicronemia) can cause atherosclerosis*** <---one of the rare occasions dogs can get this like ppl
hypercholesterolemia vs hypertriglyceridemia
chol is inc HDL, but tri is inc LDL, VLDL, hyperchylomicronemia
**what is the SCREENING test for hypoT?
Total T4 (if normal, prolly not hypoT)
when do you do total T4?
screening test. (if normal, prolly not hypoT)
what are the two DIAGNOSTIC hypoT tests?
(1) Free T4 by equilibrium dialysis *best test for free T4- sensitive and specific (2) TSH test
arguibly best test to dx hypoT?
FREET4 by equilibrium dialysis (TOTAL T4 is the screening test)
although rarely done...what are the provocative tests?
TSH Stimulation test, TRH Response test
2 thyroid panels you can do?
(1) tT4, fT4, TSH, Tab (<--that is Ab) (2) Thyroglobulin autoantibody (Positive in ~50% of hypothyroid dogs <--bc 50% is lymphocytic. Other half is idiopathic)
**what do you think of the total T4 comes back ELEVATED in a dog? What do?
NOT HYPERT. PROLLY HYPOT. Has to do with the way the test is run- total T4 is radioimmunoassay (use Abs to detect Ags, or in this case T4) and uses anti-T4 Abs. So if there are already anti-thyroid Abs (such as in lymphocytic thyroiditis or sthing) it interferes with the way the test is run and will falsely elevate the results (falsely inc number of gamma particles bound in the tube). If this happens, do a FreeT4 and/or a TSH test (diagnostic tests)
What is euthyroid sick syndrome?
Systemic illness (from any cause) can decrease tT4! 60% of dogs with severe illness of any kind had low tT4
what drugs can possibly falsely lower T4 values?
Steroids, phenobarb, abx, furosemide
what is L-thyroxine (Levothyroxine)? how does it work? Why is the risk for iatrogenic hyperT low?
used for treating hypoT. It's a synthetic thyroid hormone that is chemically identical to thyroxine (T4). It normalizes T3 and T4 and the risk of causing hyperthyroidism is low bc 4 to 3 conversion preserved: bc thyroid hormone path: T3 is freeT, T4 is transport...all go into cell, hit receptors, turn T4 into T3 (Active intracellular) or can convert to RT3 if too much T4. RT3 is the garbage product and is dumped out.
how often do you generally give L-thyroxine?
usually 2x day, some you can get down to 1x. w/in 1 wk= improvement in activity. w/in 4-6wk coat is better. neuro, cardio, hyperpigmentation may take months
how should you monitor with your L-thyroxine therapy?
supplementing with thyroid hormone, you can do total T4 (cheaper test than free T4) so just check and see if in normal range. Just make sure not too high
which CAT can have congenital hypoT?
Abyssinian (and DSH)
what is going on in FELINE hypoT?
RARE, Non-thyroidal illness. Often iatrogenic when tx hyperT
most common endocrinopathy of cats in the northern hemisphere is hyperthyroid.
Is it a thyroid or pituitary prob with cats usually?
FUNCTIONAL THYROID. Adenomatous hyperplasia/adenoma (too any cells making too much, or adenoma). Can be one or both sides.
which tumor usually is gonna cause hyperT? which is more rare?
USUALLY adenoma. Rarely thyroid carcinoma (not as malig as in dogs)
explain how CKD and hyperT are related
HyperT CONTRIBUTES TO CKD. It can CAUSE kidney dz! This is bc it causes hypertension (inc thyroid hormone= inc BP) which inc GFR which can lead to sclerosis.
How come if you start to treat hyperT, the cat didn't have kidney probs before, but now it does?
bc hyperT causes inc in renal hemodynamics and may be sorta txing the problem a little bit by inc the GFR in the dzed kidney, but there was already a problem. Not to mention the hyperT can still make it worse if the BP continues to be too much. So ideally you just tx the HyperT AND the CKD
most common signalment of hyperT?
Middle aged to old cats, no breed or sex predisposition.
how would you describe the temperament of a hyperT cat?
twitchy, dont feel good, and could have been nice but then got super grumpy.
CS/PE of a hyperT cat
Weight loss, PU/PD, +/-Polyphagia, V/D, +/- hyperactivity, weakness, dyspnea/panting, blindness/retinal hge. There will be a palpable thyroid (thyroid slip), systolic murmur, tachycardia, gallop rhythm, they are aggressive, and have a Unkempt appearance (dont feel good-- dont groom)
top 3 CSs of hyperT
weight loss, polyphagia, and palpable goiter
what will CBC/chem look like with hyperT?
CBC: erythrocytosis, inc MCV, leukocytosis, lymphopenia, eosinopenia (STRESS LEUKOGRAM). CHEM: elevated... ALT, ALP, LDH, AST, glucose, azotemia, phosphorus, bilirubin (so kidney are liver are on your list. remember hyperT is a MULTISYSTEMIC DZ)
Tachycardia, Increased R wave, Ventricular arrhythmias, Conduction abnormalities. It looks a LOT like HCM, including on echo. But this is thyrotropic cardiomyopathy.
how can you tell if it's HCM or thyrotoxic cardiomyopathy?
basically tx the hyperT-- if it gets better, it was the thyrotropic one. If it doesnt, it's HCM or too far gone thyrotoxic
what will radiographs of hyperT look like?
Mild to severe cardiomegaly, pleural effusion/pulmonary edema
why does thyrotoxic cardiomyopathy happen?
unlear why, but increased CO b/c inc tissue metabolism and o2 requirements, volume overload, reversible with therapy if early enough
can you dx hyperT with a TOTAL T4?
Actually yes-- unlike dogs with hypoT, where many things can cause T4 levels to be low, only hyperT can cause elevated T4.
What if cats have hyperT and euthyroid sick syndrome at the same time (since multisystemic dz) and you do a total T4 to see if they are hyperT?
result might be in upper half of normal. so if have CSs of hyperT with T4 in upper half of normal-- its hyperT still (bc with clinical signs would never be in upper half of normal, would have neg feedback to make it low). free T4 high and TSH would be off. low.
so what are results you can get with a total T4 which would all indicate its hyperT?
either high or in Upper ½ of normal (if resting, and in non-thyroidal illness) <--see other card for more explanation
tests for hyperT?
TOTAL T4 can be diagnostic. Can also do a Free T4 and TSH but they are more expensive. Technically there are provocative tests (T3 suppression test, TRH stimulation, TSH respone) but she has literally never had to do these
what is a Technetium/Pertechnetate scan?
test looking for ectopic thyroid tissue. Thyroid tissue starts up at heart level so can be up that whole path. and also determine if bilateral- bc cant sx if bilateral. So can determine if pt is surgical candidate. Useful for dx cats with no slip. Useful for dx metastasis. A normal thyroid should be about the same brightness on this test as the salivary glands.
the choices of tx and how to choose...
methimazole or I31 or sx....all cost about the same. So unilateral with sx can be out of hospital in 2 days and have CSs away. if bilateral do methimazole. if cant do that, I-131
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