Hyperthyroidism was diagnosed in a 14-year-old intact female leopard gecko, Eublepharis macularius, with clinical signs of anorexia, diarrhea and shedding more frequently than normal. Diagnostic workup was unremarkable except total thyroxine (T4), by radioimmunoassay, which was 20.59 nmol/L, which was elevated compared to normal snake (0.21–6.06 nmol/L) and iguana (2.97–4.65 nmol/L) values. A repeat T4 5 weeks later was 64.35 nmol/L. Normal total T4 values on six unrelated leopard geckos ranged from 6.05–19.31 nmol/L with a mean of 12.48 nmol/L. A SPECT computed tomographic (CT) technetium scan (1 mCi injected intracoelomically, scanned at 15 min) using a pinhole collimator revealed two discreet symmetric foci of activity in the ventral cervical soft tissue of a control leopard gecko, compared to a unilateral midline cervical mass, consistent with an enlarged thyroid gland, and no second focus of activity, in the patient. SPECT CT findings were consistent with a unilateral thyroid gland enlargement resulting in negative feedback and suppression of the contralateral thyroid gland. Iodine-131 (0.1 mCi) was injected subcutaneously in the ventral neck, thyroid values at 1 month (14.16 nmol/L), 3 months (6.44 nmol/L) and 5 months (12.61 nmol/L) were normal compared to other leopard geckos. Diarrhea resolved post-treatment; the gecko resumed feeding and gained weight.