We have studied hereditary congenital goiter in an inbred strain of goats. On a normal diet, the goats were hypothyroid and iodide taken up by the gland was released rapidly in the form of iodinated macromolecular material. This resulted in a low thyroidal iodine pool. An extremely low amount of thyroglobulin-related antigens (12 microgram/g tissue; normal, 100 mg/g tissue) was detected in the goitrous gland by RIA. Addition of 1 mg I-/day to the diet made the goats euthyroid and the serum protein-bound iodine increased to high values. Even under these conditions, however, the amount of thyroglobulin-related antigens was not significantly elevated (19 microgram/g tissue). On sucrose gradient, these antigens sedimented in the 7S region. No thyroglobulin-related antigens sedimented at 12S or 19S. Besides iodoalbumin, a heterogeneous group of abnormal iodoproteins containing T3 and T4 was observed with this high iodide intake and may have accounted for the hormone production. The abnormal iodoproteins had a molecular weight of 300,000-500,000 and a sedimentation value of about 7S. From these findings, we conclude that these goats were unable to synthesize thyroglobulin but with excess iodide, there was sufficient formation of T3 and T4 in the abnormal iodoproteins to make the animals euthyroid.