A group of normal and congenitally goitrous Merino sheep were investigated to identify the metabolic defect present in the abnormal animals. 2. Protein-bound iodine concentrations of serum from goitrous animals (average 5.7mug./100ml.) were higher than normal (average 4.2mug./100ml.; P 0.001), but the hormonal iodine measured as butanol-extractable (131)I was low in the serum of goitrous (average 40.3% of protein-bound (131)I) compared with that of normal (84.2%; P 0.02) sheep. The non-hormonal iodine of the serum of goitrous sheep appeared to include iodotyrosines and iodinated protein. 3. Starch-gel-electrophoretic separations of sera from normal and goitrous sheep after (131)I injection (100-500muc) showed no qualitative differences in the radioactivity of protein components. No significant differences in thyroxine-binding in vitro by serum proteins of normal and goitrous sheep were observed. 4. The clearance rates of (131)I-labelled iodotyrosines (t((1/2)) 1.2-2.9hr.) and iodothyronines (t((1/2)) 33.5-47.4hr.) were similar in normal and goitrous sheep. 5. The concentration of circulating thyroid-stimulating hormone was significantly higher (P<0.01 in three sheep, P<0.05 in one sheep) in goitrous sheep. 6. The congenital goitre appears to be due to compensatory hypertrophy of the gland resulting from an inability to synthesize an adequate supply of thyroid hormone.
