Canine hypothyroidism and dermatology: cutaneous signs, the sick-euthyroid (NTIS) confounder, and the differential diagnosis

Narrative technical review of the dermatological manifestations of canine hypothyroidism and the risk of its overdiagnosis. The work describes hypothyroid dermatosis —non-pruritic bilaterally symmetrical truncal alopecia or "rat tail," dry and dull coat, hyperkeratosis, hyperpigmentation, myxedema, and secondary infections— and its mechanism. It positions non-thyroidal illness syndrome (NTIS) as the principal diagnostic confounder: systemic illness and stress lower thyroxine (T4) reversibly without primary thyroid disease, so that an isolated low total T4 leads to unnecessary lifelong levothyroxine treatment. A multiparametric differential-diagnosis guide is proposed —clinical picture, cTSH, free T4 by equilibrium dialysis, and re-testing after recovery— to distinguish primary hypothyroidism from NTIS. Narrative review without primary data; peer-reviewed sources and a reference text verified at their original source. Clinical content: Dr. Jessica Ysabel Camacho García (CMVP 12434, ORCID 0009-0002-6837-5311).