Page 111 - Clinical Small Animal Internal Medicine

10 Feline Hyperthyroidism 79

Three studies were designed to determine the magni­ treatment of hyperthyroidism. The presence of underly­
VetBooks.ir tude of iodine control necessary to return newly diagnosed ing CKD may affect the prognosis; one study docu­
mented a shorter survival time in hyperthyroid cats with
cats to a euthyroid state; the maximum level of dietary
iodine that maintains cats in a euthyroid state; and the
vival of cats that developed azotemia with those that did
effectiveness of a therapeutic food formulated based on the azotemia. However, two recent studies comparing sur­
previous studies to control naturally occurring hyperthy­ not after treatment of hyperthyroidism found no signifi­
roidism in cats. In summary, results of these studies cant difference between the two groups if cats did not
demonstrated that a food with 0.17 or 0.32 ppm iodine become hypothyroid post treatment.
(DMB) maintained normal thyroid hormone concentra­ The reported occurrence of azotemia after treatment
tions in hyperthyroid cats, helping to further define the of hyperthyroidism ranges from 15% to 49%. Iatrogenic
range of iodine effective for managing hyperthyroidism. hypothyroidism has been reported to decrease GFR in
After confirming the diagnosis and performing a thor­ human patients. Posttreatment iatrogenic hypothyroid­
ough patient evaluation, nutritional management should ism has been reported in cats after radioiodine therapy
be discussed along with other options for managing and bilateral thyroidectomy, which constituted the pre­
hyperthyroidism. If selected as the management option, dominant therapeutic modalities in previous studies.
gradual transition to the limited‐iodine food over at least In one recent study, cats with iatrogenic biochemical
seven days is recommended. It is very important to hypothyroidism were almost twice as likely to develop
counsel owners so they understand that success of nutri­ azotemia post treatment as euthyroid cats. The hypothy­
tional management depends on the limited‐iodine food roid cats with azotemia had shorter survival times than
being the sole source of nutrition for their cat. those without azotemia, whereas, consistent with previ­
The first recheck evaluation should be done four weeks ous reports, there was no difference in survival times of
after completing the transition to y/d Feline (i.e., once euthyroid cats with or without azotemia.
the cat has eaten y/d exclusively for four weeks) and as a It is not possible to consistently predict which cats will
minimum should include physical examination and develop overt CKD after treatment of hyperthyroidism
measurement of T4, BUN, serum creatinine, and urine or have progression of their kidney disease. This should
specific gravity. All cats should have decreased T4 con­ be considered when deciding on treatment options, par­
centrations compared with baseline and many will have ticularly those that are irreversible (thyroidectomy, radi­
returned to normal by the four‐week evaluation. Clinical oactive iodine). Regardless of the option selected for
improvement including weight gain, improved hair coat, managing hyperthyroidism, it is important to remember
and decreased tachycardia/cardiac murmur also may be that the only intervention shown to improve quality of
noted by the first evaluation. Clinical signs should con­ life and prolong survival time in cats with naturally
tinue improving by the next reevaluation at eight weeks occurring CKD is feeding a therapeutic renal food. Until
and most cats will be euthyroid. Some require slightly the recent availability of limited‐iodine food, nutritional
longer to become euthyroid but it is expected that 90% recommendations have not generally been considered
will have normal T4 concentrations if the limited‐iodine for hyperthyroid cats without azotemia. In cats with
food is their sole source of nutrition. compromised renal function but without azotemia (IRIS
If euthyroidism is not achieved within 4–12 weeks, a Stage 1), the decrease in GFR associated with normaliz­
thorough history is indicated to confirm that only the ing serum T4 levels may be sufficient to prevent effective
limited‐iodine food is being fed. clearing of protein metabolic by‐products (BUN and cre­
atinine) when dietary intake of protein and phosphorus
is high. This could contribute to the occurrence of post­
Managing Hyperthyroid Cats with Concurrent therapy azotemia in hyperthyroid cats.
Kidney Disease
Chronic kidney disease (CKD) and hyperthyroidism are
more likely to be diagnosed in older cats so it is not sur­ Prognosis
prising that many hyperthyroid cats have CKD. Untreated
hyperthyroidism complicates the diagnosis of CKD Although older studies report survival times of two years
because it is associated with increased glomerular filtra­ after diagnosis, more recent data show that cats without
tion rate (GFR) and therefore often masks biochemical concurrent CKD have a median survival of 5.3 years.
markers of CKD. Regardless of the therapeutic modality Untreated hyperthyroidism is a progressive disease that
(methimazole, surgical thyroidectomy, or radioiodine), can lead to significant morbidity and mortality. Morbidity
decreased GFR, increased serum urea and creatinine and mortality in well‐managed hyperthyroid cats are
concentrations, and development of overt clinical signs more strongly influenced by the presence and severity of
of kidney disease have been reported after successful the co‐morbid disease than the hyperthyroidism itself.