Hyper and Hypothyroidism Flashcards
what is the hypothalamic pituitary thyroid axis
- hypothalamus –> TRH
- pituitary –> TSH
- thyroid gland –> thyroid hormones
what is measured in a total T4
protein bound and free unbound fraction
only the free fraction (<1%) is biologically active
congenital hypothyroidism
primary or secondary hypothyroidism in puppies resulting in deficient thyroid hormones
causes poor neurologic development + bone growth
clinical signs of congenital hypothyroidism
- normal weight at birth then failure to gain weight
- disproportionate dwarfism
- dull and lethargic
- soft, fluffy “puppy hair coat”
euthyroid sick syndrome
decreased TT4 caused by non thyroid related illness
sickness causes:
- decreased TSH secretion
- decreased T4 synthesis
- decreased thyroid binding globulins
- inhibition of T4 to T3 conversion
should testing for hypothyroid be performed on a sick animal
NO - wait until the animal feels better b/c may be artificially low due to ESS
what causes hypercholesterolemia and hypertriglyceridemia in hypothyroidism
decreased lipid metabolism by thyroid hormones
can you diagnose hypothyroidism based off of a TT4 alone
NO - can only be used to rule out hypothyroidism
low TT4 can be normal in sighthounds and performance dogs
daily variation in healthy animals
decreased with certain drugs
free T4 test
not a good screening test - more expensive and less sensitive than total T4
BUT less affected by non thyroidal illness and drugs
what is the use of TSH testing
use in conjunction with TT4 or fT4 testing to evaluate for primary hypothyroidism
can NOT be used to ID secondary hypothyroidism or hyperthyroidism b/c measurements are inaccurate at low concentrations
what is the best course of action if suspect hypothyroid dog has a normal TT4 and TSH
retest T4 and TSH in a week
can do a TSH response test but not commonly done
ddx for normal TT4/fT4 with increased TSH
early/subclinical hypothyroid
recovery from ESS
ddx for low TT4/fT4 with normal TSH
euthyroid sick syndrome
possible hypothyroidism
normal variation (sighthounds, drug induced)
ddx for low TT4/fT4 with increased tsh
hypothyroidism
drugs that mimic hypothyroidism
when checking a peak TT4, how long after administering the pill should you take the sample
4-6 hours
what is the most common cause of hyperthyroidism in cats
bilateral benign thyroid adenoma or adenomatous hyperplasia
hyperthyroidism hepatopathy
hepatocellular hepatopathy characterized by an increased ALT
important to differentiate from methimazole liver toxicity
hyperthyroid treatment induced hepatopathy
liver toxicity characterized by hyperbilirubinemia
caused by methimazole - MUST discontinue medication if patient becomes hyperbilirubinemic
what is the gold standard test for hyperthyroidism
elevated TT4
may not be best for early hyperthyroidism or non thyroidal illness (concurrent ESS will normalize TT4)
can retest in a few days if suspected hyperthyroidism when TT4 is high/normal
effect of exogenous T3 on T4 levels in a hyperthyroid cat
T4 will remain elevated due to lack of suppression
effect of exogenous T3 on T4 levels in a euthyroid cat
T4 will decrease due to negative feedback
how does hyperthyroidism mask and progress CKD
high T4 causes:
- increased CO –> increases GFR –> decreases creatinine (hides ckd)
- decreased muscle mass —> decreased creatinine (hides ckd)
- increased glomerular capillary pressure –> progression of CKD
how does treating hyperthyroidism cause azotemia
it does NOT cause azotemia - only unmasks it
treating hyperthyroid will cause GFR to decrease back to normal –> creatinine rises –> develops azotemia
NO good way to predict which cats will become azotemic after treatment
how does methimazole work
inhibits thyroid peroxidase to inhibit synthesis of thyroid hormones
side effects of methimazole
GI signs - anorexia, vomiting, diarrhea
- can continue medication
discontinue if:
- facial excoriations
- hyperbilirubinemia
- bone marrow toxicity
