A case of primary hypothyroidism associated with leishmaniasis is described in a four-year-old, male Yorkshire terrier. Clinical diagnosis of hypothyroidism was confirmed by a low baseline serum tetraiodothyronine (T4), a reduced response to thyroid-stimulating hormone (TSH) stimulation, an increased serum TSH concentration, and scintigraphic thyroid gland examination. Examination of a thyroid biopsy showed many Leishmania amastigotes, both inside and outside of macrophages, together with signs of follicular atrophy.