Minocycline and the Thyroid: Antithyroid Effects of the Drug,and the Role of Thyroid Peroxidase in Minocycline-Induced Black Pigmentation of the Gland

Minocycline (MN), a member of the tetracycline family of antibiotics, is known to induce a black discoloration of the thyroid in several species, including humans. Antithyroid effects of MN have also been reported. The aim of the present study was two-fold: (1) to determine whether thyroid peroxidase (TPO) is involved in the MN-induced black thyroid, and (2) to obtain information on the effect of MN on TPO-catalyzed iodination and coupling in model systems containing highly purified TPO. Treatment of MN with TPO in the presence of the H₂O₂ generating system, glucose-glucose oxidase, resulted in the formation of a black product (or products). In phosphate buffer, pH 7.0, the color intensity reached its peak in about 90 min. Control samples without TPO showed little or no color change during this interval. Formation of the black product(s) did not require the presence of iodide. Other members of the tetracycline family were not oxidized to dark products by the TPO system. These results provide definitive evidence that TPO is involved in the MN-induced black thyroid. MN is an inhibitor of TPO-catalyzed iodination in model systems, with a potency comparable to that of MMI and PTU. At low drug concentrations (∼25 μM), MN appeared to act as a competitive inhibitor, as previously shown for lower concentrations of MMI and PTU. However, when the drug concentration was increased, MN and the thioureylene drugs inhibited iodination by different mechanisms. With PTU and MMI, iodination was irreversibly inhibited through inactivation of TPO. However, inhibition of iodination by MN (100 μM) was not associated with inactivation of TPO and was at least partially reversible. The most potent inhibitory effect of MN was on TPO-catalyzed coupling. This was demonstrated both in a coupling test system, designed to measure coupling in the absence of iodination, and in an iodination system, in which iodination and coupling occurred simultaneously. In both systems, MN was several times more potent than PTU and MMI, or other tetracycline drugs. Based on the potent antithyroid effects of MN observed in our in vitro studies, it seems advisable to monitor thyroid function in patients receiving long-term MN therapy.