GRAVES’ DISEASE WITH CONCOMITANT MYASTHENIA GRAVIS

IMPROVEMENT OF POST-RAI HYPOTHYROIDISM AFTER THYMECTOMY

Abstract

INTRODUCTION
There is a known association between autoimmune thyroid disease and myasthenia gravis (MG) with shared autoimmunity, and treatment of one condition can affect the other. We describe an interesting case where thymectomy resulted in improvement of the thyroid hormone profile.

CASE
A 14-year-old female presented with irritability, insomnia, ophthalmopathy and a diffuse goiter was diagnosed with Graves’ disease and started on treatment. She had minimal improvement of her symptoms, with persistent subclinical hyperthyroidism. A year later she had worsening muscle weakness with diplopia and fatigability. Diagnosis of MG was confirmed with positive anti-cholinesterase-antibody and the presence of a thymoma on the CT Scan of the Thorax. Treatment with pyridostigmine improved myopathy and diplopia temporarily.

After 18 months of treatment for hyperthyroidism, she underwent radioactive-iodine (RAI) therapy at a dose of 15 mCi due to persistent biochemical hyperthyroidism with mood disturbances and intermittent muscle weakness. She was rendered hypothyroid within 5 months after RAI. Following initiation of L-thyroxine and normalization of her thyroid function, her MG also improved and pyridostigmine was discontinued. However, she developed persistent hypothyroidism a few months later, with worsening constipation and depression with suicidal ideation despite increasing doses of L-thyroxine. A year after stopping pyridostigmine, she had a flare of MG necessitating resumption of pyridostigmine at higher doses. However, with worsening constipation and difficulty in controlling both her hypothyroid and myasthenic state attributed to reduced absorption of pyridostigmine, she underwent thymectomy 4 years after the diagnosis of MG. Following thymectomy, her MG and hypothyroidism improved markedly, with reduction in constipation and improvement of thyroid function tests.

CONCLUSION
We illustrate a case of concomitant autoimmune thyroid disease and MG, describing both the ‘see-saw’ and reverse ‘see-saw’ relationship. Interestingly, treating MG with thymectomy resulted in better control of post-RAI hypothyroidism, which is postulated to be due to the improvement in gut motility and subsequent absorption of medication.