Cassava (Manihot esculenta Crantz), a widely used common vegetable of Indian origin contains dietary goitrogens. Goitrogenic content, in vitro anti-thyroidal activity and in vivo antithyroid potential of fresh and cooked cassava of Indian origin in albino rats with and without iodine supplementation were evaluated in this study. In vitro inhibition of thyroid peroxidase (TPO)activity was found with fresh and cooked cassava extract but the presence of extra iodide in the incubation media had shown reduction in such inhibition to an extent. Inhibition constant (IC50) and PTU equivalence of fresh cassava were 42.5 ± 1.35μg and 2.21 respectively. After consumption, cyanogenic constituents in cassava are metabolized to thiocyanate and iodine-retaining capacity seems to be dependent on thiocyanate exposure. In cassava fed rats, the urinary iodine concentration resembled the state of adequate iodine nutriture but thyroid gland fails to utilize available iodine. Increased thyroid weight (P < 0.001), inhibited thyroid peroxidase (TPO) activity and reduced thyroid hormone profiles (P < 0.001) were noted in fresh and cooked cassava fed rats, resembling a relative state of morphological as well as biochemical hypothyroidism even in presence of adequate iodine. © Copyright 2006 - African Journal Tradional, Complementary and Alternative Medicines.