OBJECTIVE: To study the effect of adequacy of thyroid hormone replacement therapy on coronary atherosclerosis.

DESIGN: Retrospective cohort study of elderly hypothyroid patients with coexisting coronary artery disease. The association between the adequacy of thyroid replacement and the progression of angiographic coronary artery disease was investigated. Fisher's exact test was used for statistical analysis.

SETTING: Coronary angiographies were performed at the Cardiac Catheterization Laboratory of the Victoria General Hospital, Halifax, Nova Scotia, the only tertiary referral centre for Nova Scotia and Prince Edward Island. Information about the past and current thyroid status of the subjects was collected from their family physicians.

PATIENTS: Of 4103 patients admitted for coronary angiography during 1992 and 1993, 25 were on thyroid replacement therapy to treat hypothyroidism. Ten patients who underwent more than one coronary arteriography were selected (seven females and three males, mean age 65±10 years).

RESULTS: Of five patients inadequately treated for hypothyroidism, all demonstrated angiographic evidence of coronary atherosclerosis progression. However, five of seven who were treated adequately did not show atherosclerosis progression (P=0.02, OR=0.72, 95% CI 1.36 to infinity). Decreasing or maintaining the dose of L-thyroxine at 100 mg or less resulted in coronary atherosclerosis progression in six of six patients, whereas five of six patients taking fixed or increasing doses of L-thyroxine 150 mg or higher were spared from disease progression (P=0.015, OR=0.41, 95% CI 2.4 to infinity).

CONCLUSIONS: Angiographic coronary artery disease progression may be prevented by adequate thyroid replacement in hypothyroidism. With the help of modern, sensitive thyroid stimulating hormone assays higher doses of L-thyroxine may be safer and more effective in the atherosclerosis management of this patient population. Thyroid hormones can protect against atherosclerosis, presumably due to their metabolic affects on plaque progression.