Hypothyroidism Flashcards
Physiologic effects of thyroid hormones
Increase metabolic rate
Catabolic effects on muscle and adipose tissue
Cardiac - increased contractility and HR
Stimulates erythropoiesis
Regulates cholesterol synthesis/degradation
Normal growth of neuro/skeletal system (mental alertness, peripheral nerves in adults)
What is the difference between primary and secondary hypothyroidism?
Primary - decrease in T3 and T4; common
Secondary - decrease in TSH; uncommon
Three causes of primary hypothyroidism
Decrease in T3 and T4
- Lymphocytic destruction (“thyroiditis”)
- Idiopathic atrophy
- Bilateral neoplasia
Signalment for hypothyroidism
Usually middle aged dogs
Some breeds affected more often
Common clinical signs of hypothyroidism
Metabolic
Dermatologic - symmetric alopecia, hyperpigmentation, otitis, seborrhea, poor coat quality, myxedema (causes sad face)
Neurologic - respond to levothyroxine; peripheral, cranial and CNS
Cardiovascular - bradycardia; decreased beta receptors
Reproductive dysfunction
Ocular - lipid issues
Megaesophagus, laryngeal paralysis
Features of congenital hypothyroidism
Decreased cognitive ability
Stunted and disproportionate growth
Large, broad head with short, thick neck
Short limbs
Abdominal distention
Macroglossia
Hypothermia
Delayed dental eruption
Hypothyroidism in cats is usually iatrogenic during:
Treatment for hyperthyroidism (I-131 or Methimazole)
Should you test a dog for hypothyroidism who has no clinical signs of the disease?
NO
A suspicion of hypothyroidism must initially be based off what three things?
History
Clinical signs
Physical exam findings
Name two diseases that have overlapping clinical signs with hypothyroidism.
Atopic dermatitis
Hyperadrenocorticism (Cushing’s)
Potential findings in a dog with hypothyroidism include:
- Normocytic, normochromic, nonregenerative anemia
- fasting hypertriglyceridemia
- fasting hyper cholesterolemia
- increased hepatic enzymes
What is the gold standard diagnostic for hypothyroidism?
TSH-stim test
Very expensive to perform; may not be feasible
Expectations for response to treatment of hypothyroidism
Clinical signs, improved activity 1-2 weeks
Weight loss 8 weeks
Neuro signs, hair coat several months
Side effects of levothyroxine treatment for hypothyroidism
Hyperthyroidism - PUPD, polyphagia, tachycardia, nervousness
Lack of response/apparent tx failure
- inaccurate dx
- inadequate dose/compliance
- poor absorption (GI disorder)
Treatment monitoring for hypothyroidism:
Recheck _____ four weeks after any dose change.
Once well regulated, check every ______.
T4
6 months
Drug of choice for treating hypothyroidism
Levothyroxine
*half dose in cardiac patients (increases myocardial O2 demand)
*food may decrease oral bioavailability
_______ is a good screening test and a good place to start with thyroid testing.
Total T4
*highly sensitive; low T4 will identify MOST hypothyroid patients; false positives common - normal variations
How should you treat a dog with decreased total T4, normal to decreased free T4, and normal TSH?
This is a normal physiologic response and does not require supplementation.
During illness there is a decrease in thyroid response to TSH leading to activation of T4. TX is to correct the underlying illness.
If you find a dog has low total T4, what is the next step before starting thyroid supplementation?
Additional testing:
Free T4
- active form; must be measured by Equilibrium Dialysis
- less affected by non-thyroidal illness
or
TSH
- will be high in animals with hypothyroidism without T3 and T4 being produced
- can be influenced by drugs, illness, pituitary exhaustion
- normal TSH does NOT rule out hypothyroidism
A high ____ combined with a low _____ is diagnostic for hypothyroidism.
High TSH
Low T4
*normal TSH does NOT rule out hypothyroidism
Presence of ______ antibodies is supportive but not diagnostic for hypothyroidism.
Anti-thyroglobulin
Anti-T3
Anti-T4
