Hyperthyroidism is a multisystemic metabolic disorder that occurs as a
result of excessively high concentrations of thyroid hormones in the
bloodstream. It is the most common glandular disorder in cats. Untreated
hyperthyroidism results in increased cardiac output, and increased blood
flow to the kidneys. Although the disease is treatable, in recent years
it has become apparent that previously undetected kidney disease may
suddenly be unmasked, and that known kidney disease may worsen in some
cats after correction of their hyperthyroidism.
The disease is caused by excessive secretion of thyroid hormones by
hyperplastic (overly large), usually benign thyroid glands. Although
extensively studied, it is still not known why some cats develop
hyperthyroidism while others do not.
There is no breed or sex predisposition for the disorder, but it occurs
almost exclusively in older cats. Less than 6 percent of cases are
younger than 10 years old. The average age of onset is 12 to 13 years.
The impact of the disease can be significant, as hyperthyroidism can
affect many body systems, especially the heart.
What to Watch For
Excessive drinking and urinating
Hyperactivity (restlessness, excessive vocalization)
Complete blood count
Thyroid hormone level
Antithyroid medication (tapazole)
Possible cardiac medication
Surgical removal of the thyroid glands (rarely done anymore)
Radioactive iodine therapy
Hill’s Prescription Diet Y/D
Consequences involving the kidneys can be minimized or prevented
in hyperthyroid cats by assessing kidney function before undertaking
therapy to treat the hyperthyroidism. Cats are then given oral
medication to bring the thyroid hormone levels back into the normal
range slowly, temporarily controlling the condition. Kidney function
is then re-evaluated. If kidney function remains stable, a more
permanent method of therapy (surgery, radioactive iodine) can be
undertaken. If kidney function worsens, the veterinarian is faced
with the dilemma of balancing the two disorders so that the most
severe clinical signs of either disorder are kept to a minimum.
Home Care and Prevention
Consistently administer any prescribed anti-thyroid and/or cardiac
medication if necessary. Treat for kidney failure (subcutaneous
fluids, prescription diets, phosphate binders, gastric protectants,
vitamin D supplements) if warranted and prescribed.
It is still not understood why some cats develop hyperthyroidism
while others do not. Thus, there are no measures that can be taken
to prevent the development of hyperthyroidism.