Thyroid Supplementation: Monitor Carefully to Avoid Thyrotoxicosis

Hyperthyroidism increases the risk of atrial fibrillation independent of the source of the thyroid hormone. Depressed thyroid-stimulating hormone (TSH) levels indicate hyperthyroidism even if thyroid hormones are within the normal range. Therefore overtreatment of (especially subclinical) hyperthyroidism increases the risk of atrial fibrillation.

The journal Thyroid Function and Dysfunction has published a study that shows that treating community-dwelling elders too zealously for elevated TSH levels is a leading cause of thyrotoxicosis in its July 2015 issue.

The “Baltimore Longitudinal Study of Aging” (run by the National Institute on Aging) follows healthy older community residents without cognitive impairment, functional limitations, and/or major chronic conditions. One objective of this study is periodic monitoring of thyroid hormone and TSH levels. This is the first prospective study of US thyrotoxicosis rates.

The researchers found a rate of hyperthyroidism was 1.5 cases per 1000 patient-years in patients untreated with thyroid hormone supplementation. It was considerably higher—17.7/1000 patient-years—among treated patients. The adjusted hazard ratio was 27.5 for women and 3.8 for men.

Most patients in this study were treated with levothyroxine alone.

The rate of thyrotoxicosis was more than 10 times as likely in patients receiving supplementation than the background rate in a matched untreated population. In the US, medical thyroid hormone supplementation causes half of thyrotoxicosis cases; in numbers, this is an additional 240,000 cases annually.

Two participants received desiccated thyroid preparations (which are T3/T4 combination hormones) and one took cytomel (T3 alone) with levothyroxine as part of their therapy. All 3 of these patients developed thyrotoxicosis.

The researchers discussed lab results with patients at study visits, yet over-treatment with thyroid supplements continued for years in 30% of patients. The study authors assert that overtreatment was either deliberately continued because patients and clinicians thought it improved quality of life, or because they simply did not appreciate the risk. Overtreatment was more likely to continue in patients was being treated for concomitant fatigue or depression.

Clinicians should assess the possibility that patients are over-treated for hypothyroidism at initiation of thyroid supplementation and periodically thereafter to minimize serious adverse events such as atrial fibrillation. Female sex and age over 80 years are the 2 most important risk factors. White race, and higher BMI characteristics were also associated with increased the rate of thyroid hormone supplementation.