Geriatric cats are prone to hyperthyroidism and chronic renal disease (CRD). Because thyroid function can influence renal
function, it's not surprising that these conditions frequently coexist. Hyperthyroidism can, in fact, mask CRD because of
the impact of this disease on cardiovascular function. There are general increases in cardiac output (higher heart rates,
increased contractility) and peripheral vasodilation (increased metabolic demand resulting in the release of vasodilators)
that lead to an increase in renal blood flow and glomerular filtration rate.
With treatment of the hyperthyroid state, these effects are lost, which can cause an increase in renal values on blood work.
In one study, about 15 percent of hyperthyroid cats that had normal renal values became azotemic within 240 days of treatment.1 In cats that are azotemic before treatment, the CRD can worsen substantially.
Treatment for hyperthyroidism can occasionally lead to a hypothyroid state that may or may not be clinical. This is most common
with bilateral thyroidectomy (100 percent) and radioactive iodine (75 percent) and is much rarer with methimazole treatment
Past research finds cause and effect
One study found that hypothyroidism resulted in a shorter lifespan in cats treated for hyperthyroidism.3 This study had two portions: to follow 12 cats that were treated with radioactive iodine to see if they became hypothyroid,
and to look at a separate group of cats six months after treatment for hyperthyroidism to see if there was a relationship
between thyroid status and the development of azotemia.
Of the 12 cats treated with radioactive iodine, 10 became hypothyroid, as diagnosed via an elevated thyroid-stimulating hormone
(TSH) concentration. In the survival portion of the study, 51 percent of the 80 cats evaluated had a total thyroxine (T4) concentration below the reference interval. Of these cats, 68 percent had a TSH concentration above the reference range,
classifying them as hypothyroid. Five cats had normal T4 but high TSH concentrations, which excluded them from the analysis.
Hypothyroid cats had lower heart rates, lower packed cell volumes and lower alkaline phosphatase (ALP) activity than euthyroid
cats. There was no difference in survival times between the euthyroid and hypothyroid groups. However, 57 percent of hypothyroid
and only 30 percent of euthyroid cats developed azotemia after treatment, which was statistically significantly different.
The survival for azotemic hypothyroid cats was also significantly shorter (a median of 456 days) than that of nonazotemic
hypothyroid cats (a median of 905 days) and euthyroid cats (a median of 794 days).
This study suggests that cats treated for hyperthyroidism that are azotemic and hypothyroid may be at greater risk of a poorer
outcome. Additional thyroid function testing such as a TSH concentration to assess for subclinical hypothyroidism may be a
consideration. The assay used in this study was a commonly used canine assay (Immulite Canine TSH assay—Diagnostic Products
Corporation) since a feline assay is not widely available. Check with your laboratory for additional information on this type
Reversing the effects
At the 2012 ACVIM Forum in New Orleans, the Royal Veterinary College in London presented a research abstract on the impact
of restoring euthyroidism on renal function in cats made iatrogenically hypothyroid.4 Cats were selected that had a T4 concentration below the reference interval and an elevated TSH concentration. These cats were being treated medically, so
doctors reduced their hyperthyroid medications to establish euthyroidism.
A total of 12 cats were included in the study. Of these 12, nine were azotemic. Once euthyroidism was established, creatinine
concentrations dropped significantly and azotemia resolved in three of the nine cats. Other statistically significant changes
included an increase in ALP activity and heart rate with a decrease in body weight. This study did not address if there was
any survival benefit to this dosage adjustment, though it would be expected.