Great question! Even people with adrenal problems often make a little bit of cortisol. This little bit can be enough to keep them from getting very sick. In hypothyroidism the body's need for cortisol goes down and the little bit you make lasts longer. So if a person has both thyroid and adrenal disease, treating the thyroid without treating the adrenal problem first causes the little cortisol to run out quicker.
Increased catabolism. Good question. It is believed that thyroxine supplementation will increase urinary cortisol excretion in addition to stimulating the enzyme that converts cortisol, the active form, to cortisone, the inactive form. The opposite is true of this fascinating phenomenon as there had been several case reports of hypothyroidism bein reversed with steroids in individuals with ai.
Adequate replacement. Thyroid hormone speeds up metabolism and therefore puts more stress on adrenal glands if they're not making enough cortisol to begin with. As long as you are getting adequate steroid replacement for the adrenal insufficiency, adding any necessary thyroid hormone replacement will be ok.