Levothyroxine Dose May Differ for Obese Women After Thyroidectomy

NASHVILLE, Tenn. — Women with obesity who undergo total thyroidectomy may require a different treatment regimen than those without obesity, researchers reported at the American Association of Clinical Endocrinologists (AACE) 24th Annual Scientific & Clinical Congress.

Specifically, they noted that, after thyroidectomy, women with obesity may require a higher levothyroxine (LT4) dose per ideal body weight but not per total body weight than those without obesity.

In their study, the researchers found that there was significant heterogeneity in LT4 dosage requirements among obese women. Consequently, they believe it may be best to use the ideal body weight as opposed to the total body weight as a more initial approach for therapy of hypothyroidism in a majority of obese women in order to prevent iatrogenic hyperthyroidism.

“The current studies only looked at patients who were normal weight or overweight. They did not look at the obese. There is some suggestion that obese patients may be different in how they metabolize drugs. There may be a difference in pharmacokinetics,” said endocrinologist Kathleen Glymph, MD, who is with the University of Tennessee Health Science Center in Memphis.

Because the current recommendations for LT4 replacement in patients with hypothyroidism are based on the evidence from studies in nonobese patients, clinicians are left with inadequate guidance, according to Glymph. She said clinicians should not use the same approach for obese women because they often require a different LT4 regimen in order to achieve euthyroidism. 

Glymph and her colleagues retrospectively identified consecutive women who were treated at an endocrinology clinic between November 2010 and October 2014. All the patients had undergone total thyroidectomy for benign goiter or stage I thyroid cancer and achieved euthyroid state on LT4 therapy after surgery.

For this investigation, the researchers defined obesity as BMI greater than 30. The investigators used Student’s t-tests and Fisher’s exact tests to analyze clinical and biochemical characteristics.

Data from 30 hypothyroid women (mean age, 48.7 years) were included in the analysis. Among the 30 patients, 14 were nonobese and 16 were obese. This cohort was composed of 17 white and 13 black patients. There were no differences in age, thyroid-stimulating hormone (TSH) and ideal body weight between groups. As expected, though, obese women had higher total body weight (105.1 kg vs. 65.3 kg; P<.001) and BMI (38.8 vs. 24.6; P<.001).

Results showed that obese women required higher LT4 dose, as compared with nonobese women (148 mcg vs. 102 mcg; P=.0002). Although there was no difference in LT4 per total body weight (1.43 mcg/kg vs. 1.60 mcg/kg; P=.21), LT4 per ideal body weight was found to be higher in obese vs. nonobese women (2.65 mcg/kg vs. 1.89 mcg/kg; P=.0006). This difference still held even after adjustments for age and race (P<.05).

The researchers found a marked variability in LT4 dose in obese patients. Ten obese patients became euthyroid on LT4 dose of less than 150 mcg, and six became euthyroid on LT4 dose greater than 150 mcg. Age, weight and BMI were not significantly different between these subgroups, but those who became euthyroid on the higher dose were more likely to have postoperative hypoparathyroidism (50% vs. 0%; P=.04); a higher LT4 dose per total body weight (1.73 mcg/kg vs. 1.26 mcg/kg; P=.06); and a higher LT4 dose per ideal body weight (3.27 mcg/kg vs. 2.27 mcg/kg; P=.002).

“When we looked at the subgroup analysis, there was a big range in what they needed of LT4, regardless of the same BMI,” Glymph said in an interview with Endocrinology Advisor. “It is possible to overdose or underdose these patients, and the problem is there are no randomized data on what to do with these patients. There is a lot more that needs to be done, and we hope this will lead to randomized trials in the future.”

Reference

1. Glymph K et al. Abstract #1019. Presented at: American Association of Clinical Endocrinologists (AACE) 24th Annual Scientific & Clinical Congress; May 13-17, 2015; Nashville, Tenn.