Clinical significance of autoimmune thyroid disease in myasthenia gravis

We investigated thyroid function and autoimmunity in 74 (61% females) consecutive patients with MG. 30 of these patients were tested twice: the time between the investigations ranging from 1 to 4 years. MG was diagnosed on the basis of typical clinical symptoms, a positive tensilone-test, and/or detectable acetylcholine antibodies and/or repetetive stimulation tests. Eye involvement was present in 86%, concurrent thymomas in 56%. The following parameters were measured in the serum of these patients by commercially available kits: free and total T4, TSH, TSH-Receptor-Antibodies ("TRAB"), antibodies against thyroglobulin (Tg-Ab), against thyroid microsomes (M-Ab) and against acetyl-choline-receptors . An age matched group of 50 patients (54% females) with no known thyroid disease from a cardiological ward and from the neurological outpatient department served as control. There was only 1 MG patient with overt thyroid dysfunction (iodine induced thyrotoxicosis in a patient with autonomous adenoma and no circulating thyroid autoantibodies detected at the second investigation). There were, moreover, 1 euthyroid MG-patient on L-thyroxine therapy with a history of Hashimoto's disease and positive thyroid autoantibodies and 1 other MG-patient with mildly elevated TSH without elevated thyroid antibodies, who has had subtotal thyroidectomy without substitution therapy years before for unknown reasons. Tg-Ab were positive (>360 IU/ml) in 5%, M-Ab were positive (>154 IU/ml) in 15% of the MG patients. The control group had 4% Tg-Ab and 6% M-Ab. TRAB levels were normal in all patients and controls. The relative increase in M-Ab frequency was not statistically significant (x-square test). We conclude from our results, that autoimmune thyroid disease may be associated with MG but that the occurrence of thyroid dysfunction induced by autoimmunity is a very rare phenomenon in MG.