# Black thyroid in a dog on long‐term doxycycline therapy

## TL;DR

A dog on long-term doxycycline developed black thyroid, a rare condition previously seen in humans, highlighting the need for awareness to avoid unnecessary surgery.

## Contribution

First reported case of black thyroid in a dog caused by doxycycline therapy, emphasizing the importance of recognizing this benign condition.

## Key findings

- Black thyroid pigmentation was observed in a dog on long-term doxycycline therapy.
- Histopathology confirmed the presence of brown pigment in thyroid follicular cells, similar to human cases.
- No evidence suggests that thyroidectomy is necessary for black thyroid in dogs.

## Abstract

To increase awareness of black thyroid in dogs and to prevent unnecessary total thyroidectomy. A benign condition called “black thyroid” has been documented in greater than 250 people on chronic minocycline therapy, and rarely in animals. To our knowledge this is the first report of black thyroid in an animal secondary to doxycycline therapy.

Case report.

One 10 year‐old female spayed Collie‐cross dog.

A dog on long‐term doxycycline underwent a right parotid sialoadenectomy and left thyroidectomy to remove associated tumors. Black pigmentation of both thyroid lobes was observed intraoperatively. The left thyroid gland and associated nodule were excised, leaving the right lobe intact.

Histopathology of the left thyroid nodule and right parotid salivary gland were consistent with thyroid follicular‐compact cell carcinoma with metastasis. Finely granular brown pigment was present multifocally within the cytoplasm of many of the thyroid follicular cells and extracellularly within the colloid as irregular gray to brown glassy aggregates. The pigment was negative for iron and calcium and had minimal to no immunoreactivity for melanin.

These findings aligned with those reported for the condition black thyroid in humans. At this time, there is no evidence that performing a thyroidectomy is necessary or appropriate for black thyroid. Veterinary surgeons should be aware that dogs on long‐term doxycycline therapy may have this discoloration, so unnecessary total thyroidectomy can be prevented in affected animals.