High cholesterol levels may be due to genetic factors, poor dietary habits, or some underlying disorder (referred to as secondary hypercholesterolemia).

I have always checked to see if my patients have a treatable cause of secondary hypercholesterolemia before placing them on a diet or medication. Because an underactive thyroid gland (hypothyroidism) is the most common treatable cause of elevated cholesterol, it is always a good idea to test for hypothyroidism and to treat it when present.

The usual screening test for hypothyroidism is a high level of thyroid-stimulating hormone (TSH), which is secreted by the pituitary gland. Recognizing hypothyroidism by the low blood levels of thyroid hormone, the pituitary responds by increasing its release of TSH in an effort to stimulate the thyroid to produce more hormone.

Some years ago, however, one of my patients with severe hypercholesterolemia reminded me that hypothyroidism can be present despite a low or normal TSH. I started him on a cholesterol-lowering statin when his TSH was in the low normal range. Several months later he began to complain of headaches and visual difficulties.

An imaging study of his brain showed a tumor of the pituitary gland, and further tests revealed low blood levels of pituitary hormones and thyroid hormone. His hypothyroidism was not accompanied by an elevated TSH because the tumor had interfered with the secretion of TSH. In fact, the low TSH levels were responsible for his hypothyroidism.

His tumor was removed by surgery, and his hypercholesterolemia was cured when his statin was stopped, and he was treated instead with thyroid hormone.