Approximately one dog in 42 will have a special form of Addison’s disease. To understand this form, we have to add some more details about the adrenal cortex. We have already explained that the adrenal cortex makes corticosteroids: the glucocorticoids that control sugar, fat, and protein use during stress and the mineralocorticoids that control electrolytes (mainly sodium and potassium) during stress.
The adrenal cortex has three layers: the zona fasciculata, the zona reticularis, and the zona glomerulosa. The inner two layers make the glucocorticoids and the outer zona glomerulosa layer makes the mineralocorticoids.
Most dogs get Addison’s disease when all three layers of the adrenal gland are destroyed and no corticosteroid hormones of any kind can be produced. With atypical Addison’s disease, the problem is limited to the layers that produce the glucocorticoids. This creates a patient who cannot regulate blood sugar normally but who is not at risk for an Addisonian crisis.
Diagnosis is still done with the ACTH stimulation test.
Treatment consists of supplementing glucocorticoid hormones, such as prednisone. Often these patients ultimately progress to the more typical Addison’s disease, complete with electrolyte imbalance.
A similar deficiency in glucocorticoids (but not mineralocorticoids) results when a pet has been on long-term oral glucocorticoids (such as prednisone) and medication is discontinued too abruptly. Long term glucocorticoid use leaves the outer layers of the adrenal cortex with nothing to do (and no stimulation from the pituitary gland since pills or shots are providing the body with more than enough glucocorticoids. Once the medication is withdrawn, the body is back to relying on its own adrenal glands for glucocorticoids but the gland has atrophied from lack of stimulation. This creates a deficiency in glucocorticoids similar to atypical Addison’s disease and is the reason why steroid hormones are typically tapered off rather than abruptly discontinued. True atypical Addison’s disease can be distinguished from overuse of medication by a plasma ACTH level (high in atypical Addison’s and low with medication overuse).
