Description: Thyroid gland dysfunction was studied in Zebu cattle experimentally infected with Trypanosoma vivax, based on the levels of serum thyroxine (T4) and occurrence of pathological lesions at 13 weeks post infection(p.i.). There were statistically signififi cant declines in the T4 levels in the infected group at 3 weeks p.i. (P<0.05), 6 weeks p.i. (P<0.01) and from 9 to 11 weeks p.i. (P<0.05); when compared to the normal T4 levels that were observed in the uninfected, control bulls throughout the period of study. The thyroid glands of both infected and control bulls showed no gross lesions and the mean weights were not statistically signififi cant (P>0.05)between the groups. Histopathological lesions observed in the thyroid glands of the infected bulls were focal in occurrence and include; squamous metaplasia of follicular epithelium with fifi broplasia and widened interfollicular connective tissue. The control bulls showed no histopathological lesions in the thyroid glands.It was concluded that thyroid gland dysfunction occurs in trypanosomosis due to T. vivax in cattle and it is suggested that this could contribute to the weakness associated with trypanosomosis