emergency thyroid disease Flashcards
Can you summarise the normal pathophysiology of the thyroid gland
- Hypothalmus produces TRH
- TRH stimulates anterior pituitary to produce TSH
- TSH stimulate thyroid to produce and release T4 and T3
- T4 and T3 have negative feedback on the pituitary inhibiting TSH
How are T3 and T4 transported? summarise their actions
- they are highly plasma protein bound
- lipid soluable, allowing entry to cells to alter gene expression
- main effects: increase metabolism, protein catabolism, increased BP and GFR, INCREASED EXPRESSION OF BADRENERGIC RECEPTORS
In what situations might acute cahanges in thyroid hormone lead to thyroid toxicosis
- radioactive iodine treatment or thyroid surgery
- agressive thyroid palpation
- Abrupt cessation of antithyroid drugs
- drugs or illness that reduce hormone binding to plasma proteins (increased free)
- Concurrent illness increasing sensitivity to thyroid hormones
What is the significance of b-adrenergic receptors
- Hyperthyroid cats have increased expression of b-adrenergic receptors
- stress resulting in catecholamine release can pericipate an emergency crisis.
What are the most common co-morbidities in feline thyroid disease
hypertension, CKD and HCM
What are the possible clinical signs of feline thyroid toxicosis
fever, agitation GI signs, arrthymias and chf
- severe may also have noss of limb fuction or neuro signs from thrombi or vascular injury
how is thyroid toxicosis diagnosed?
- no lab test need hyperthyroid cat, associated clin signs and a precipitating event
- an elevated T4 conc can be used to diagnose hyperthyroidism
What is the focus of treatment in a thyrotoxocosis crisis
- focus on sequalae, systemic support and management of comorbitities
- beta blockade wit propanolol (or esmolol or atenolol) Propanolol may also reduce peripheral conversion of T4 to T3.
- Cautious amlodipine if betablockade does not reduce hypertension (care, renal)
- if no CHF then judicious IVFT
Why might a patient with thryrotoxocosis be tachypnoeic
- agitiation, fever
- pocus to rule out CHF, consider further imaging if not resolving
What is the justification for b vitamin administration to cats in thyroid crisis
Thiamine deficiency is common
How are the high levels of circulation throid hormone managed?
Not as much a priority but still needed
- methimazole, higher end of dose range 5mg PO BID.
- Administration of iodine after this can cause a paradoxical decrease in thyroid horomone synthesis
- potassium iodine 25mg PO Q8 or contrast medium IV can be used
- care with IV contrast, potential nephrotoxicity
What clinical signs can be seen with acute hypothyroid crisis in dogs
- Mental depression weakness and bradycardia (common)
- hypotension and hypothermia
myxoedema may be seen
What labratory findings are noted in a hypothyroid crisis
- not specific
- hypercholestrolaemia and an midl non regenerative anaemia.
- T4 and fT4 are low wuth TSH usually elevated
What is the most common precipitaing factor for a hypothyroid crisis?
Bacterial infection
How is hypothyroid crisis treated
- supportive warming and perfusion
- deal with the underlying/preciptating event
- levothyroxine 0.02mg/kg PO q 12 should be administered and lead to resolution in 1-2 days
What other conditions have been associated with hyothyroidism that can present as emergencies
Gall bladder mucocele, peripheral neuropathy, thromboembolic disease.
(eg peripheral neuropathy as a cuse for asp pneumonia or laryngeal paralysis)
Why is a disgnosis of hypothyroidism delayed in patients presenting with critical illness
- many different drugs and non thyroidal illnesses can reduce the T4 concentration
- may see a reduction in the plasma proteins which bind T4 in many illnesses
- Elevated TSH with Low T4 is supportive but not 100% sensitive/specific, best to delay testing
