I n a small - scale sheep flock located in the State of Mexico with eight ewes and one stud, four ewes had clinical signs of endemic goiter , t wo aborted fetuses were used for a diagnostic stud y . In fetuses, thyroid gland samples were used for histopathological evaluation, and liver, heart, kidney and skeletal muscle samples were used to measure selenium. In the ewes the blood serum profile of thyroid hormones was measured by ELISA test. In fetuses t here was an increment of thyroid gland size (6 - 8 cm), firm consistency and red intense color , as well as s carce fleece and subcutaneous edema ; t he re w ere hyperplasia and hypertrophy of follicular cells of thyroid, with cube - like aspect, diffuse expanded, irregular shape and distension with pres ence of vacuoles and abundant eosinophilic colloid. The selenium concentrations in liver and kidney of fetuses were low (0.25 and 0.90 ppm). In ewes, t he thyroxin (T 4 ) and triiodothyronine (T 3 ) contents were deficient (0.48±0.08 and 4.14±0.29 μg dL - 1 ) and the ratio (T 4 :T 3 ) was very broad (1:8.6 μg dL - 1 ). It is concluded that the lambs suffered from colloidal goiter; thyroid hormones concentra t ions in ewes were low and confirmed a thyroid metabolism altered ; additionally goiter may be aggravated by selenium deficiency.