43 Hyperpigmentation due to hypothyroidism
Hyperpigmentation is a frequent presentation in canine skin disease and is most commonly the result of inflammation, due to parasitic, bacterial, fungal or allergic skin disease. Hormones also exert an effect on the pigmentary system, although their mode of action is not well understood. This report describes a case of hypothyroidism that resulted in marked ventral hyperpigmentation.
Overall, the history of pruritus and signs of papules, scaling, erythema, lichenification and hyperpigmentation were most suggestive of an inflammatory skin disease. Ventral hyperpigmentation is a common presentation in hypersensitivity disorders. The multifocal dorsal alopecia could have been due to a staphylococcal folliculitis, or perhaps self-trauma that the owners were unaware of. However, the hair loss had a bilaterally symmetrical pattern suggestive of an endocrinopathy, which can also be a factor in the development of hyperpigmentation. There was no historical evidence of signs suggestive of hyperadrenocorticism and hypothyroidism was considered the most likely endocrinopathy to be involved. Therefore, the differential diagnoses included the following: