MON-400 - Non-Pigmentated Papillary Thyroid Carcinoma in the Setting of Minocycline-Associated Black Thyroid

Background
Long-term minocycline use has been associated with black thyroid, attributable to interaction between minocycline and the thyroid peroxidase enzyme. Despite concerns, a causal relationship between minocycline use and thyroid cancer has not been established. Here, we describe a case of non-pigmented papillary thyroid carcinoma (PTC) occurring in the setting of minocycline-associated black thyroid.
Case
A man in his late 50s with a history of heart transplant, type 2 diabetes mellitus, CKD stage 3b, hypertension, and long-term minocycline use (100 mg twice daily for >4 years) underwent PET/CT scan (from skull vertex to mid-thigh) for lung nodules, which incidentally revealed a hypermetabolic right thyroid lesion. Thyroid ultrasound showed a right lobe nodule classified as TIRADS 3. Fine-needle aspiration (FNA) demonstrated Bethesda category IV cytology, and molecular analysis revealed NRAS and TERT c.124C>T mutations, indicating a 75–90% risk of malignancy. Total thyroidectomy showed a diffusely black thyroid with a normally colored 4.5-cm nodule. Pathology confirmed minimally invasive follicular-variant PTC measuring 4.5 cm, confined to the thyroid gland, with black pigment present in surrounding follicles and histiocytes. The patient tolerated the procedure well and was started on levothyroxine 100 mcg for post-surgical hypothyroidism.
Discussion
Minocycline, a tetracycline antibiotic commonly prescribed for acne and other chronic infections, can cause pigment deposition within thyroid colloid and follicular epithelial cells through interaction with thyroid peroxidase. Black thyroid remains predominantly a surgical diagnosis, as FNA does not reliably detect the characteristic pigmentation preoperatively. Lack of pigmentation in PTC is likely related to differential metabolism within tumor cells. Although a definitive causal relationship has not been established between minocycline use and thyroid carcinoma, an increased incidence of PTC has been reported in minocycline-treated patients with black thyroid. Incidence of PTC among patients with minocycline-associated black thyroid has been reported at 30% to 39%. Thus, there should be a high index of suspicion for thyroid carcinoma in patients presenting with a thyroid mass while chronically taking minocycline.

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