Autoimmune Thyroid Disorders in Autoimmune Addison Disease
A nationwide registry-based study including 442 patients with Addison's disease (AAD) and autoimmune thyroid disease were identified through the Norwegian National Registry of Autoimmune Diseases. Of 912 registered AAD patients, 442 (48%) were diagnosed with autoimmune thyroid disease. A total of 380 (42%) had autoimmune hypothyroidism. Of the 203 with available thyroid function tests at time of diagnosis, 20% had overt hypothyroidism, 73% had subclinical hypothyroidism, and 7% had thyroid levels in the normal range. Negative thyroid peroxidase antibodies was found in 32%. Ninety-eight percent were treated with levothyroxine, 5% with combination therapy with liothyronine or thyroid extracts, and 1% were observed without treatment. Seventy-eight patients (9%) were diagnosed with Graves disease (GD), of whom 16 (21%) were diagnosed with autoimmune hypothyroidism either before onset or after remission of GD. At the end of follow-up, 33% had normal thyroid hormone levels without antithyroid-drugs or levothyroxine treatment. The remaining had either active disease (5%), had undergone ablative treatment (41%), or had developed autoimmune hypothyroidism (21%).

The true prevalence of hypothyroidism in AAD is lower than reported in the current literature. Careful consideration of the indication to start thyroxin therapy is warranted. Long-term remission rates in GD patients with AAD are comparable to recent reports on long-term follow-up of patients without AAD.