Conn syndrome or primary hyperaldosteronism can either be caused by an adrenal adenoma that is secreting aldosteron or by idiopathic hyperaldosteronism which is presumed to due generalised hyperplasia.
To differentiate between the two a postural aldosterone test is conducted. In hyperplasia the normal physiology still works which means that the aldosterone level should increase in the upright position while in adenoma such a response is not there.
The distinction is important as the treatment modality is usually surgical in the case of adenoma while it's medical in the case of hyperplasia.
See this related thread also;
Hypertension + Adrenal Mass