Histopathologic diagnosis of thyroid neoplasia in teleosts is complicated, because thyroid tissue is unencapsulated, and normal tissue can have wide ectopic extensions. Assessment of thyroid hormone concentrations in conjunction with histologic evaluation of thyroid carcinoma has not been reported in teleosts, even though routinely performed in other species. A yellowbar angelfish (Pomacanthus maculosus) presented with a mass that extended beyond the gill arches. Partial surgical resection was performed, and a histologic diagnosis of thyroid carcinoma was made. Plasma from the affected fish and two control Pomacanthus were used to assess thyroid hormone concentrations using a domestic mammalian assay. Thyroid-stimulating hormone and throxine were undetectable in two of three fish. Triiodothyronine was lower in the affected fish than in controls. The tumor did not appear to be actively secreting hormone. This is the first characterization of thyroid tumors and corresponding thyroid hormones in teleosts, which may assist in diagnosis of this disease.