Hyperthyroidism as the cause for ‘brittle' Addison's disease
KM Evans & DE Flanagan
A 40-year-old woman was admitted for investigation of severe fatigue, abdominal pain, anorexia and myalgia over the preceding year. Past medical history included vitiligo, coeliac disease and depression. Initial investigations revealed hyperkalaemia, normocytic anaemia and hypercalcaemia with an inappropriately normal parathyroid hormone level. Low TSH with normal free T4 and T3 (0.02 milliunits per litre, 17.1 picomoles per litre and 6.3 picomoles per litre respectively) were attributed to concomitant illness; repeat testing after one month was planned. Hypoadrenalism was suspected and confirmed with a short synacthen test: serum cortisol 79 nanomoles per litre at 0min, 61 nanomoles per litre at 60min. This was felt to be Addison's disease, rather than secondary hypoadrenalism, in view of elevated ACTH levels (461 nanograms per litre) and the presence of adrenal antibodies in the serum. She was discharged on hydrocortisone 30/10/30 mg and fludrocortisone 100mcg daily.
Over the following months, her hydrocortisone dose was progressively weaned. Although no objective weight loss or signs of hypovolaemia were noted, she continued to experience ill-health. A cortisol day curve was performed, showing adequate peaks at one-hour post dose, but low cortisol levels pre-dose, indicating rapid hydrocortisone metabolism. Further investigation revealed overt hyperthyroidism: free T4 45.5 picomoles per litre, TSH less than 0.01milliunits per litre.
Since achieving euthyroidism with carbimazole therapy, a day curve has indicated adequate replacement on hydrocortisone 20/10/10mg. It is possible that there is a degree of malabsorption, secondary to coeliac disease, contributing to her greater-than-average hydrocortisone requirement.
In summary, this lady exhibits primary hypoadrenalism, followed almost immediately by the development of hyperthyroidism. Thyrotoxicosis is known to be a hypermetabolic state, however, it is unusual for hyperthyroidism to first manifest as 'brittle' Addison's disease.