Dyslipidemia is an established and major risk factor of atherosclerotic cardiovascular disease (ASCVD). Dyslipidemia can be caused by inherited genetic disorders or can result from metabolic derangements. Conf irming the cause of dyslipidemia is crucial, as intervention can substantially reduce the risk of ASCVD. Here we present a 44-year-old woman who visited our outpatient department due to extremely elevated plasma levels of total cholesterol and low-density lipoprotein cholesterol (LDL-C). Familial hypercholesterolemia (FH) was initially suspected. However, extensive biochemical examination eventually confirmed severe hypothyroidism as the cause of dyslipidemia. After thyroxine supplementation therapy, her thyroid function returned to normal, and her lipid profiles returned to normal range without the need for lipid-lowering therapy (LLT). This case report underscores the importance of a comprehensive evaluation of secondary causes of dyslipidemia before initiating LLT.