UpToDate に以下の記述があります。
■The presentation of ACTH deficiency is almost exclusively that of the resulting cortisol deficiency. There are two important clinical distinctions between cortisol deficiency due to ACTH deficiency (secondary adrenal deficiency) and cortisol deficiency due to adrenal disease (primary adrenal insufficiency) with a secondary increase in ACTH release

•ACTH deficiency does not cause salt wasting, volume contraction, and hyperkalemia because it does not result in clinically important deficiency of aldosterone.
•ACTH deficiency does not result in hyperpigmentation.

鉱質コルチコイドは保たれている事が多いので高カリウム血症は来さないようです。
また、下垂体前葉機能が低下する病態でもACTHは生命の維持に一番大切で、最後まで残る傾向が強く他の系統に比べ、保持されやすいです。