Feline Hypothyroidism Flashcards
What is the most common cause of hypothyroidism in cats?
IATROGENIC - thyroidectomy, radioiodine, anti-thyroidal drugs
(not commonly seen with thyroid (y/d) diets)
What is the most common result in cats following radioiodine therapy?
will be transiently hypothyroid for about a month, then resolve and become euthyroid within 3-6 months
Why is hypothyroidism following hyperthyroid treatment so significant?
iatrogenic hypothyroidism causes cats to more likely become azotemic due to decreased GFR —> decreases survival times!
What are the most common clinical signs associated with iatrogenic hypothyroidism in cats? Why are they hard to recognize?
- weight gain
- decreased appetite
- lethargy
+/- dermatologic changes
signs are expected with resolution of hyperthyroidism and usually not apparent at the time of diagnosis
What is the screening test of choice for diagnosing feline hypothyroidism?
total T4
(alone does not confirm hypothyroidism - non-thyroidal illness (CKD) can make euthyroid cats look hypothyroid!)
What TSH levels are expected in hypothyroid cats? How does this test differ compared to total T4?
increased TSH
- less affected by non-thyroidal illness
- diagnosis reached with concurrent low T4 levels
How are patients monitored following radioiodine and methimazole treatment? What is monitored?
RADIOIODINE = 1 mo, 3 mo, 6 mo
METHIMAZOLE = every 2-4 weeks
- resolution of hyperthyroidism
- detection of hypothyroidism
- assessment of renal function
How are hypothyroid cats medically managed? When should they be rechecked? What is expected?
Levothyroxine without food (1 hr before meal or 3 hours after)
recheck T4 and TSH within 1 month —> T4 should be within reference range with a decrease in TSH
How is survival of hypothyroid cats increased?
improve kidney function by supplying thyroid hormones (Levothyroxine) to increase GFR
What is the goal in treating cats that become hypothyroid on Methimazole? How can this be achieved?
mid-range T4 levels —> 1.5-3
decrease Methimazole dose by 25-50% (check TSH if concerned for non-thyroidal illness)
What is the monitor schedule following hypothyroisidm caused by radioiodine treatment?
1 month - monitor, likely to be experiencing transient hypothyroidism so treatment is not necessary quite yet
3 months - investigate with T4 and TSH +/- other basal thyroid hormones and if consistent with hypothyroidism, can treat or wait a few months if not experiencing azotemia
6 months - treat regardless of presence of azotemia
What is the difference between overt and subclinical hypothyroidism? How does this change treatment plan?
OVERT = decreased T4 with increased TSH; monitor and treat at 3 and 6 months
SUBCLINICAL = normal T4 with increased TSH; monitor based on degree of azotemia (or if it is worsening)
In what 3 ways can feline hypothyroidism be prevented?
- lower Methimazole dose
- delay treatment with radioiodine if TSH is not suppressed
- lower dose of radioiodine
