Scales in a dog after levothyroxine sodium treatment: A case report.

Background: Scales are composed of exfoliated keratinocytes and are frequently a secondary change, requiring the determination of the underlying cause. Consequently, making an appropriate differential diagnosis of scales and determining treatment necessity based on the results is crucial in dermatological practice. Case Description: We present a 7-year-old, neutered male Toy Poodle examined at Myogadani Animal Hospital with chief complaints of exercise intolerance, unexplained weight gain, and dermatological signs, including symmetrical alopecia of the trunk. The dog was diagnosed with hypothyroidism and administered levothyroxine sodium. A significant number of scales were observed, primarily on the trunk, after 1 month of administration. The scales disappeared after only reducing the dose of levothyroxine sodium for 3 months. Additionally, hair regrowth was observed as the scales disappeared. In this case, the scales were primarily observed in the alopecia area of the trunk, where hair growth was subsequently noted. Hence, levothyroxine sodium administration was thought to not only facilitate hair regrowth but also influence skin turnover, causing the appearance of scales. Conclusion: Clinicians who treat hypothyroidism need to be cognizant that symmetrical scales, primarily located on the trunk, do not co-occur with other skin lesions, such as erythema, and are not itchy, may appear within a month. The scales will naturally disappear in such cases when hair growth is complete; thus, no additional treatment is required.