Quitting smoking as a probable trigger for new-onset hypothyroidism after successful medical treatment of Graves' disease: case report

Graves' disease (GD) is the most common cause of hyperthyroidism while Hashimoto or autoimmune thyroiditis is the most common cause of hypothyroidism. Spontaneous hypothyroidism may develop after successful medical treatment of GD in up to 20% of cases. This report presents a gentleman who is a known smoker and was diagnosed with GD at the age of 64 years. He was counseled about smoking cessation and started with medical treatment using carbimazole (CBZ). He was adequately controlled using medical treatment, yet he continued to smoke. After 2 years of medical treatment, CBZ was stopped due to developing hypothyroidism on the minimum dose of treatment. Celebrating the discontinuation of treatment, the patient decided to quit smoking. One month later, he was euthyroid; however, 4 months later, he developed overt hypothyroidism. He received levothyroxine replacement therapy and titrated to achieve euthyroidism and remained on levothyroxine for more than 5 years. The possibility that quitting smoking may have triggered the development of hypothyroidism was raised due to the coincidence of developing hypothyroidism only 4 months after quitting smoking. Current smoking is associated with a higher risk of developing both GD and Graves' orbitopathy. Quitting smoking is associated with a higher risk of developing new-onset thyroid autoimmunity. Quitting smoking is also associated with a sevenfold higher risk of autoimmune hypothyroidism especially in the first year of smoking cessation. Involved mechanisms may include a sudden increase in oxidative stress, a sudden increase in iodide delivery to thyroid follicles, or promoting T-helper 1-mediated autoimmune thyroiditis after quitting smoking. The present case suggests that quitting smoking may be a triggering factor for the development of hypothyroidism following successful medical treatment of GD, a phenomenon that may affect one-fifth of GD patients without previously reported triggers. Quitting smoking may trigger hypothyroidism in previously treated Graves' disease patientsGraves' disease is the commonest cause of hyperthyroidism. Medical treatment is the mainstay treatment, and about 5-20% of patients may develop hypothyroidism after successful medical treatment. The triggers to this conversion are not known. The present case, a 64 years old gentleman who is a smoker, after being diagnosed with graves' disease, receives medical treatment for 2 years. On the occasion of stopping medical treatment for graves' disease, he decides to quit smoking. One month later he is euthyroid off medications, but 4 months later, he develops severe hypothyroidism, for which he receives replacement therapy for the following five years. The possibility that quitting smoking may have triggered this conversion was raised. Smoking is associated with a 2-folds higher risk of having graves' disease. Quitting smoking on the other hand increases the risk of acquiring thyroid autoantibodies, and new onset autoimmune hypothyroidism. Quitting smoking is also associated with symptoms of weight gain, constipation, and depression, all of which may also occur in hypothyroidism. That is why, ordering thyroid function tests is recommended in recent quitters if they develop such symptoms. Thus, quitting smoking in the present case may have triggered this severe hypothyroidism. Underlying mechanisms may involve increased oxidative stress or autoimmune reactions favoring the occurrence of autoimmune thyroiditis.