A new study highlights the importance of avoiding both exogenous hyperthyroidism and exogenous hypothyroidism to decrease cardiovascular risk and death among patients receiving thyroid hormone treatment.
“Our findings suggest that clinicians should make every effort to maintain euthyroidism in patients on thyroid hormone treatment, regardless of underlying cardiovascular risk, particularly in vulnerable populations, such as older adults,” senior author Maria Papaleontiou, MD, said in an interview.
Commenting on the study, David S. Cooper, MD, of Johns Hopkins University School of Medicine, Baltimore, agreed that the findings are significant.
“Both undertreatment and overtreatment were associated with adverse cardiovascular outcomes, meaning that patients’ thyroid function needs to be monitored, and levothyroxine adjusted if need be, on an ongoing basis,” he told this news organization.
Getting the balance right: a tricky task
Variations in thyroid hormone levels falling above or below target ranges are common with thyroid hormone therapy, as a wide array of factors can prompt the need to regularly adjust dosing to maintain “index” levels. And while guidelines from the American Thyroid Association (ATA) recommend maintaining serum thyroid stimulating hormone (TSH) levels in the normal ranges during treatment, the task is tricky.
“Despite these [ATA] guidelines, prior studies in adults with hypothyroidism have shown that up to 30% are undertreated and up to 48% are overtreated,” said Dr. Papaleontiou, an assistant professor in the Division of Metabolism, Endocrinology at the University of Michigan, Ann Arbor.
In a previous study, Dr. Papaleontiou and colleagues showed that the intensity of thyroid hormone treatment is a modifiable risk factor for incident atrial fibrillation and stroke, however, less is understood about the association with cardiovascular mortality.
For the new study, published in JAMA Network Open, Josh M. Evron, MD, of the University of North Carolina, Chapel Hill, and colleagues further investigated the issue in a large, retrospective cohort of 705,307 adults in the Veterans Health Administration Corporate Data Warehouse treated with thyroid hormone during 2004-2017 who had a median follow-up of 4 years.
They investigated the roles of TSH as well as free thyroxine (FT4) levels among 701,929 adults in the group with data on TSH and 373,981 patients with FT4 measurements.
The mean age of participants was 67 years and 88.7% were male.
Over the course of the study, 10.8% of patients (75,963) died of cardiovascular causes.
Compared with patients with normal thyroid levels, those with exogenous hyperthyroidism related to thyroid hormone treatment had an increased risk of cardiovascular mortality, specifically including when TSH levels were below 0.1 mIU/L (adjusted hazard ratio, 1.39) and when FT4 levels were above 1.9 ng/dL (AHR, 1.29), independent of factors including age, sex, and traditional cardiovascular risk factors, including hypertension, smoking, and previous cardiovascular disease or arrhythmia.
In addition, the increased risk of cardiovascular mortality was observed with exogenous hypothyroidism, specifically among those with TSH levels above 20 mIU/L (AHR, 2.67) and FT4 levels below 0.7 ng/dL (AHR, 1.56), after multivariate adjustment.
Of note, the risk of cardiovascular mortality was dose-dependent, with the risk increasing progressively with the lower and higher TSH levels, compared with normal levels.
The increased mortality risk in relation to TSH levels was more pronounced among older patients, compared with FT4 associations, the authors note.
“From a clinical perspective, older adults, and particularly the oldest old (aged 85 years), appear to be the most vulnerable, with increased risk of cardiovascular mortality with both exogenous hyperthyroidism and hypothyroidism,” they report.
Among key limitations is that women, who make up the majority of patients with thyroid disease, are under-represented in the predominantly male population of the Veterans Health Administration.
Nevertheless, “because the risk of cardiovascular disease is higher for men than for women, and because more than 70,000 women were included in this cohort, the results of this study are highly clinically relevant,” the authors note.