Iatrogenic hypothyroidism can occur in approximately 10% to 30% of hyperthyroid cats following radioactive iodine treatment, with studies showing considerable variations in prevalence (1%-78%; see Take-Home Messages).5-10 Considering the impact iatrogenic hypothyroidism has on kidney function and overall survival, early diagnosis is ideal. Clinical signs (eg, weight gain, lethargy) alone may not raise suspicion of hypothyroidism, as clinical signs can overlap with resolution of hyperthyroidism.4 The screening test of choice for hypothyroidism in dogs is evaluation of total T4 concentration, which is also routinely monitored in cats following treatment for hyperthyroidism. However, a total T4 concentration below reference range does not confirm hypothyroidism, as the presence of nonthyroidal illness can confound the diagnosis. This is particularly true in cats with CKD, in which serum T4 concentration is low in approximately 50% of cases.11-14 The combination of low T4 concentration and high TSH concentration is used to diagnose overt iatrogenic hypothyroidism, as seen in Betty.4 Conversely, some hypothyroid cats can have a low-normal T4 concentration; an increased TSH concentration in these cats is indicative of subclinical hypothyroidism, as seen in Isabella.4 Therefore, only monitoring T4 concentration in cats that have undergone treatment for hyperthyroidism can lead to overdiagnosis or can preclude the diagnosis of hypothyroidism. Periodic evaluations—including evaluation of T4 and TSH concentrations—at 1, 3, 6, and 12 months following radioiodine treatment are recommended. If persistent, overt iatrogenic hypothyroidism is detected, treatment with levothyroxine should be considered, particularly in cats that develop azotemia. Similarly, if hypothyroidism is detected in cats receiving methimazole therapy, the dose should be appropriately adjusted.
Pet owners should be informed of the risks and complications of iatrogenic hypothyroidism, the importance of posttreatment monitoring, and the necessity of levothyroxine supplementation in select cases following radioiodine therapy. Preventive strategies for iatrogenic hypothyroidism, particularly in cats with mild-to-moderate hyperthyroidism, can include treatment with lower doses of radioactive iodine, which has been shown to lower the prevalence of iatrogenic hypothyroidism without compromising treatment efficacy.9