Cassava meals are common in many tropical countries and are processed into different forms of human delicacies and animal feeds (Adekunle et al.2011) .It is fast becoming the major ingredient of many diets in Nigeria especially in the bakery, brewery and pharmaceutical industries. . A study was conducted to determine the effect of cassava feed formulations on thyroid functions and their relevance in clinical diagnosis. Thirty wister rats mean weight of 238±25.6g were acclimatized for one week and fed the different cassava feed formulations and water ad libitum. Thereafter, the rats were sorted out into 5 groups of six each. Group 1,2,3,4 and 5(control) were fed on 30g each of garri, cassava flour, tapioca, fresh cassava and grower’s mash (local rat diet) respectively. After 4 weeks, each animal was exposed to 5mls of chloroform in cotton wool and anaesthetized by inhalation. Blood samples were collected by cardiac puncture and then assay for tetra-iodothyronine (T4), tri-iodothyronine (T3) and thyroid stimulating hormone (TSH). The data obtained were presented in tables and the significance determined at p<0.5 using paired samples t-test. In comparison with the control, group1 shows a significant decrease in T3 and T4 and group 2 shows a significant increase in T4 and TSH p<0.05. Group 3 shows a significant decrease in level of TSH p<0.05 while in group 4, the increase in T3 and T4 and the decrease in TSH had no statistical significant difference p>0.05.It was concluded that garri and cassava flours lower thyroid functions (goitrogens) while Tapioca and fresh cassava increase serum T3 and T4 activities and could worsen cases of hyperthyroidism.