BACKGROUND
Hyperthyroidism is a condition where the thyroid gland produces high levels of thyroid hormone. This can result in various symptoms, such as weight loss, tremors, anxiety, a fast heart rate, and palpitations. Subclinical hyperthyroidism is considered a milder form of hyperthyroidism. In subclinical hyperthyroidism, the levels of thyroid hormones (T3 and/or T4) are within the normal range, but the TSH level is below normal. This suggests that there is an excess of thyroid hormone in the body even though thyroid hormone levels are normal in routine laboratory tests.

Although subclinical hyperthyroidism may not cause noticeable symptoms or the symptoms may be subtle, research has shown that it can affect the heart muscle and increase the risk of heart problems such as irregular heart rhythms, heart failure, and death. The heart muscle contains receptors for thyroid hormones, establishing a well-known connection between thyroid disease and heart disease. Despite this, limited studies have investigated the heart effects of subclinical hyperthyroidism in patients with preexisting heart disease.

In the present study, the authors specifically examined the association between subclinical hyperthyroidism and outcomes in patients with preexisting heart disease who underwent a cardiac stent procedure.

THE FULL ARTICLE TITLE
Yang J et al 2022. The impact of subclinical hyperthyroidism on cardiovascular prognosis in patients undergoing percutaneous coronary intervention. J Clin Endocrinol Metab 107:986–997. PMID: 34850030.

SUMMARY OF THE STUDY
The researchers examined 8,282 patients with heart disease who had a heart stent procedure at a hospital in Beijing, China, between January 1, 2013, and December 31, 2013. From this large group of patients they selected 332 individuals who had subclinical hyperthyroidism and compared them to 1,271 patients with normal thyroid function.

Subclinical hyperthyroidism was defined as having normal levels of thyroid hormones but abnormally low levels of thyroid-stimulating hormone (TSH) in the blood, specifically below 0.55 mIU/L. When the TSH level was very low i.e., below 0.10 mIU/L, it was called severe subclinical hyperthyroidism. They then followed these patients for 24 months to see if the group with subclinical hyperthyroidism had higher rates of major adverse heart events. Such events included cardiac death, non-fatal heart attack, and need for bypass grafting to a blocked artery in the heart.

After 24 months, the authors found that 11.4% of patients with subclinical hyperthyroidism and 8.8% of patients with normal thyroid function (euthyroid) experienced a major adverse heart event. However, this difference between the two groups was not statistically significant. Furthermore, when comparing patients with severe subclinical hyperthyroidism to patients with normal thyroid function, there was also no significant difference in the number of major adverse heart events. This suggests that even in cases of more severe subclinical hyperthyroidism, there is no increase in the risk of experiencing these serious heart events. Overall, subclinical hyperthyroidism was not identified as a risk factor for having a major adverse heart event in this particular group of patients.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
Doctors have been unsure whether or not to treat subclinical hyperthyroidism, especially in patients who don’t have any symptoms. This study shows that patients with subclinical hyperthyroidism and preexisting heart disease do not face a higher risk of adverse outcomes compared to those with normal thyroid function. Although the study only observed patients for 24 months (a relatively short period), these findings help us better understand the connection between subtle thyroid disease and heart complications. Therefore, perhaps not all patients with subclinical hyperthyroidism need treatment.

— Phillip Segal, MD

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