Hypothyroid Crisis Myxomatous edema

Question 1

myxedema coma is a misnomer why:

difficult to diagnose because

Which breed is at at increased risk:

What increases risk

findings observed in many, but not all, dogs:

Tx:

Subjective improvement in mentation or ambulation occurs within:

Px:

Answer 1

hypothyroid crisis often not comatose and no myxedema

rare, CS, clinpath abnormalities nonspecific

Rottweiler

Concurrent disease - infection (pneumonia)
steroids, NSAIDs, or sx

Myxedema, obesity, mental dullness, hypercholesterolemia, and nonregenerative anemia

IV levothyroxine at a dosage of 5 mcg/kg q12h is a safe and effective treatment for dogs suffering from a hypothyroid crisis.

24 to 30 hours of IV levothyroxine administration dogs

with supportive care and IV levothyroxine, good

Question 2

pathophysiology -incompletely understood

thyroid hormones regulate cell function in many organs by:

Thyroid hormones exert:

hallmarks of myxedema coma in human beings are:

Answer 2

binding intranuclear receptors and promoting expression of various enzymes

chronotropic and inotropic effects in the heart
catabolic, metabolic, calorigenic, and developmental effects in other organs

altered mental status 
inadequate thermoregulation
decreased respiratory 
cardiovascular function
concurrent disease

Question 3

MoA mental depression:

Answer 3

multifactorial and may be a result of decreased blood flow and oxygen delivery
hyponatremia, lack of a direct effect of thyroid hormone

Question 4

MoA hypothermia

Answer 4

Inadequate thyroid hormone function in the hypothalamus may result in inability to regulate body temperature

decrease in the calorigenic effect

Question 5

MoA hypoventilation

Answer 5

secondary to decreased respiratory system responsiveness to hypoxia and hypercapnia

Question 6

MoA cardiomyopathy:

increase in α-myosin heavy chains (MHC), which have decreased adenosine triphosphatase (ATPase) activity and a decrease in ____

Answer 6

T3 increase # and affinity β-adrenergic receptors to catecholamines

and a decrease in β-MHCs, which have more adenosine triphosphatase activity.

Question 7

How is concurrent dz implicated?

Answer 7

Concurrent disease may prevent normal compensatory mechanisms from responding appropriately to a hypothyroid crisis

Absence of concurrent disease in most cases of hypothyroidism may explain why myxedema coma remains rare

Question 8

Explain the development of myxedema when it does occur

Answer 8

1. secondary to accumulation of the glycosaminoglycan hyaluronic acid in the dermis
2. impaired renal perfusion secondary to decreased cardiovascular function results in inability to excrete water and contributes to development of edema
3. excessive secretion of ADH may also contribute to hyponatremia, fluid retention, and edema in some patients

Question 9

risk factors:
breed:
age:
concurrent disease:
drugs:

Answer 9

Rottie
middle aged
yes, commonly an infection (UTI, otitis, pyometra, pneumonia)
steriods lower T3concentration
NSAIDs may cause suppression of thyroid-stimulating hormone (TSH)

Question 10

C/S

do they show classic signs/what are the classic signs:

Answer 10

classic physical hallmarks
including myxedema
hypothermia
bradycardia
hypoventilation
hypotension
coma

Question 11

Labs: 
classic finding (but not all present usually)

Answer 11

mild nonregenerative anemia
hypercholesterolemia
lipemia
ALP

Question 12

MoA anemia in hypoT4

Answer 12

1: T3 binds thyroid hormone receptors on erythroid progenitors and act directly to increase erythroid proliferation
2. Thyroid hormones also increase expression of the erythropoietin gene

Question 13

MoA hypercholesterolemia:

hyponatremia and hypoglycemia are common and hypoth mech but none certain

Answer 13

decreased thyroid hormone function induces dyslipidemia are widely studied and not fully understood. Alterations in both synthesis and transport of lipids

Question 14

definitive diagnosis:

however, results often are not available immediately and treatment of a hypothyroid crisis must begin before confirmation of a diagnosis

what can interfer with thyroid hormone measurement?

Answer 14

Thyroid axis testing

low thyroxine and high TSH concentrations
some hypothyroid dogs do not have elevated TSH

Severe lipemia

Question 15

Several drugs can cause low thyroxine with or without high TSH concentrations:

Answer 15

glucocorticoids
NSAID
TMS, sulfas
phenobarbital, potassium bromide 
propranolol
lithium

Question 16

ddx for hypercholesterolemia:

Answer 16

hypothyroidism, DM, HAC, 
PLN, cholestatic disease
postprandial hyperlipidemia
primary hyperlipidemia (Miniature Schnauzers, Shetland Sheepdogs)
iatrogenic causes (glucocorticoids)

Question 17

tx

supportive:
BP:
hypoNa
temp

Answer 17

IV levothyroxine 5 mcg/kg q12h is a safe and effective
before results of thyroid axis testing are available and before the diagnosis of a hypothyroid crisis is confirmed

pressors and fluids
0.5mEq/L
no periph vasodil - blankets and ambient temp

Question 18

thyroxine (T4) rather than IV ___ safer:
hence levothyroxine
T3 adverse side effects:

Answer 18

triiodothyronine (T3)

cardiac arrhythmias, angina pectoris, and pneumonia

Mortality of humans with myxedema coma is increased with high dosages of IV levothyroxine (>500 mcg/24 hours, which is equivalent to about 7 mcg/kg/24 hou