The seeds of Descurainia sophia (L.) Webb ex Prantl contain goitrogenic glucosinolates, such as gluconapin (3-butenyl glucosinolate). Because of the important role of iodine in the synthesis of thyroid hormones and the inhibitory activity of D. sophia on iodine uptake by the thyroid gland, this study aimed to determine the effects of D. sophia syrup on clinical and biochemical variables of thyrotoxicosis in hyperthyroid patients. In this randomized, double-blind trial, 10 newly diagnosed hyperthyroid female patients were randomly assigned to treatment with (1) methimazole (MMI) plus D. sophia capsules (350 mg/d) or (2) MMI plus placebo capsules. The primary outcomes were clinical and biochemical manifestations of hyperthyroidism after 60 days. Serum levels of FT3 and FT4 significantly decreased (5.9 ± 2.5 vs. 9.4 ± 5.9 and 29.1 ± 3.1 vs. 31.77 ± 3.7, respectively) and the serum thyroid-stimulating hormone (TSH) concentration significantly increased in the D. sophia group contrasted to the placebo group (4.7 ± 0.1 vs. 0.05 ± 0.02). Significant improvement in the thyrotoxicosis clinical symptoms was reported in the D. sophia group contrasted to the placebo group (p < .05). D. sophia can suppress plasma levels of FT3 and FT4 and may be useful as adjunctive therapy for hyperthyroidism.