One hundred and forty-two persons referred for assessment of thyroid function were studied in relation to their thyroid function and fasting serum lipids. Forty-five individuals, who had no family history of thyroid disease, were euthyroid, and were negative for thyroid antibodies formed the control group. The remaining 97 patients were fitted into six groups as follows--euthyroid Hashimoto's thyroiditis, thyroiditis with exagggerated TSH response to TRH, and four groups of increasing grades of thyroid failure based on the level of serum TSH. Only in the two most severely hypothyroid groups was cholesterol significantly elevated as compared with control levels. A graded increase in mean serum triglyceride, was observed as thyroid failure increased; however, values were not different (P greater than 0.05) from the control group. However, partial correlation analysis indicated that serum cholesterol was related to increasing levels of serum TSH and that this dependency was not influenced by age or weight. Although serum cholesterol was related to thyroid function in the study population, it was an insensitive indicator of the metabolic effects of thyroid hormone.