Familial hypercholesterolaemia and primary hypothyroidism: coexistence of both disorders in a young woman with severe hypercholesterolaemia.

Severe hypercholesterolaemia in a 7-year-old girl was found to be due to the coexistence of heterozygous familial hypercholesterolaemia and chronic lymphocytic thyroiditis with primary hypothyroidism. Her cultured skin fibroblasts showed a 70-75% reduction in the binding, internalization and degradation of 125I-labelled low density lipoprotein and several other family members were hypercholesterolaemic. Replacement therapy with L-thyroxine reduced the concentrations of total and low density lipoprotein cholesterol and apoprotein B in the patient's plasma to values similar to those in other sibs with only heterozygous familial hypercholesterolaemia. Severe hypercholesterolaemia attributable to coexistent heterozygous familial hypercholesterolaemia and hypothyroidism in teenage girls may be estimated to occur with a frequency of between 1 and 500 000 and 1 in 10(6). This value is similar to the gene frequency for homozygous familial hypercholesterolaemia.