Hyperadrenocorticism is correct!
Adrenal function testing is recommended for any diabetic dog that develops insulin resistance with dermatologic signs of hyperadrenocorticism (HAC). Hypothyroidism is also a valid differential diagnosis for Rosie, especially since insulin resistance can be a feature of canine hypothyroidism. In this dog, however, the normal serum TSH concentration makes hypothyroidism less likely. In addition, half the dogs with HAC have low serum T4 or free T4 concentrations. This is thought to result from suppression of pituitary TSH secretion and the effects of HAC as a nonthyroidal illness.
For Rosie, ACTH stimulation test results indicated HAC, and low-dose dexamethasone suppression testing confirmed pituitary-dependent HAC. Rosie responded to treatment with trilostane (Vetoryl—Dechra; 30 mg orally b.i.d.). Over a few weeks, her insulin requirements fell to 1 mg/kg b.i.d. as her diabetic state became easier to control. She lost weight, her potbelly resolved, and her coat grew back.
Although never treated for hypothyroidism, her serum T4 and free T4 concentrations were both normal when rechecked after six months of treatment with Vetoryl.