OBJECTIVE: Patients with hyperthyroidism occasionally need rapid restoration to the euthyroid state. In view of the increased enterohepatic circulation of thyroxine (T4) and triiodothyronine (T3) in thyrotoxicosis, and metabolic effects of konjac glucomannan in gastrointestinal system, we aimed to determine the activity of glucomannan in treatment of hyperthyroidism. METHODS: A prospective, randomized, placebo-controlled, one-blind study design was used with newly diagnosed 48 hyperthyroid patients (30 patients with Graves' disease and 12 with multinodulary goitre). They were assigned to one of the following treatment groups: I) methimazole 2 x 10 mg, propranolol 2 x 20 mg, and glucomannan (Propol) 2 x 1.3 gr daily for two months; II) methimazole 2 x 10 mg, propranolol 2 x 20 mg, and placebo powder daily for two months. RESULTS: No differences were detected from the point of view of the baseline thyroid hormone levels between groups (p > 0.05). Further analyses revealed that the patients receiving glucomannan at the end of the second, fourth and sixth weeks of the study had significantly lower serum T3, T4, FT3 and FT4 levels than the patients who received placebo (p < 0.05). TSH was not different between the two groups at any specific time (p > 0.05). At week 8, thyroid hormone levels were not shown any differences. The glucomannan-treated group had a more rapid decline in all four serum thyroid hormone levels than the placebo-treated group. CONCLUSIONS: We believe our preliminary results indicate that glucomannan may be a safe and easily tolerated adjunctive therapeutic agent in the treatment of thyrotoxicosis. This combination therapy seems most effect during first weeks of treatment of a hyperthyroid patient.