High blood cholesterol might be a sign of hypothyroidism. Indeed, patients with hypothyroidism typically have elevated cholesterol levels because thyroid hormone regulates cholesterol metabolism. Treatment of hypothyroidism with levothyroxine promptly reduces blood cholesterol, but does it return to normal levels?

It has been assumed for years that normalization of TSH levels with levothyroxine also normalizes cholesterol levels. However, a new systematic review of almost 100 studies including thousands of hypothyroid patients treated with levothyroxine revealed that patients with normal TSH continue to have slightly elevated blood cholesterol.

Evidence that this was happening was first obtained in hypothyroid rats treated with levothyroxine. These animals continue to have higher than normal cholesterol despite normal TSH levels. Subsequently, a survey of tens of thousands of individuals randomly selected across the United States identified approximately 500 that were taking levothyroxine and had normal TSH. Remarkably, they were also more likely to be takin statin, a commonly prescribed medication to lower cholesterol levels.

The new study evaluated a series of markers of thyroid hormone action in nonpregnant adult humans treated with levothyroxine with normal serum TSH levels. An all-inclusive analysis (aka meta-analysis) showed that levothyroxine-treated hypothyroid individuals had about 3.3 mg/dL higher blood LDL and 9.6 mg/dL higher blood cholesterol compared to healthy controls. In studies that did not concomitantly assess healthy controls, both LDL and blood cholesterol levels were above the normal range, despite normalization of TSH. Other parameters known to be affected by thyroid hormones were also investigated; all were normalized by treatment with levothyroxine.

The authors concluded that not all markers of thyroid hormone action are normalized by levothyroxine at doses that normalize TSH, including blood LDL and cholesterol levels. Therefore, all hypothyroid patients, regardless of treatment, should discuss these results with their physicians and ensure that cholesterol levels are within normal range.

Contributed by Antonio C. Bianco, MD PhD

Original studies at: PMID: 30124904