Minocycline Induced Non-Autoimmune Thyroiditis

Minocycline, a tetracycline that is commonly prescribed for acne vulgaris, is known to be associated with black pigmentation of thyroid. There are only limited cases on association of minocycline and thyroid function abnormality.

Case Description:

A 19-year-old male with past medical history of allergic rhinitis was admitted to the hospital for sinus tachycardia with heart rate in 150s. He had symptoms of palpitation, diaphoresis and an unintentional weight loss of 10 pounds. He was found have thyrotoxicosis with TSH of 0.01µIU/mL (0.27-4.7µIU/mL) and free thyroxine of more than 7.77 ng/dL (0.92-1.7ng/dL). He was started on methimazole 10 mg twice a day and metoprolol tartrate 25 mg twice a day and was discharged 3 days after his symptoms improved. His Thyroid Stimulating Immunoglobulin and Thyroid Peroxidase came back negative. His thyroid uptake and scan also showed low uptake, consistent with thyroiditis. He was advised to stop his methimazole and continue on his metoprolol tartrate.

Further questions were asked to look for the etiology of his thyroiditis. He denied recent respiration infection, neck tenderness or new medications prior to his recent hospitalization. Detailed medication history revealed that he had been taking minocycline 180 mg twice a day for his acne vulgaris for over 1 year. He had stopped his minocycline since his hospitalization for thyrotoxicosis.

After stopping his minocycline, patient’s thyroid hormone function continued to improve and became euthyroid after 3 months of stopping minocycline. His heart rate significantly improved and metoprolol was titrated down.

Discussion:

Minocycline, a member of tetracycline family, is widely used to treat acne vulgaris. Minocycline is associated with black pigmentation of thyroid, first reported in 1976. There are not many cases describing minocycline associated with thyroid function abnormalities. To our knowledge, there are only a handful of reports on minocycline induced thyroiditis. We report another case of minocycline induced non-autoimmune thyroiditis. We believe that there are more cases that are not identified since minocycline is not listed as a common cause of drug induced thyroiditis. It is important for providers prescribing minocycline to be aware of such potential side effects and be able to give their patients appropriate counseling on this antibiotic. Further investigation is warranted to find out the incidence of minocycline induced non-autoimmune thyroiditis.