“Women are up to 8 TIMES more likely than men to develop thyroid problems." (Secondary claim in the same post: "According to research highlighted by Yale School of Medicine, as many as 21 MILLION of the approximately 23 million Americans taking levothyroxine may have been prescribed the medication unnecessarily," sourced in the post to…”

The headline statistic traces to the American Thyroid Association, whose press materials state: "Up to 60 percent of those with thyroid disease are unaware of their condition. Women are five to eight times more likely than men to have thyroid problems." The same ATA page states that an estimated 20 million Americans have some form of thyroid disease and one woman in eight will develop a thyroid disorder during her lifetime. The "8 times" in the post is the top of that published range, and the post retains the "up to" qualifier. Independent clinical reference data are broadly consistent for autoimmune thyroid disease specifically. For Hashimoto's thyroiditis, the leading cause of hypothyroidism in iodine-sufficient countries, the female-to-male ratio is at least 7:1 to 10:1, and incidence rises with age with most cases occurring between ages 45 and 55. The sex ratio is not uniform across all "thyroid problems." For thyroid cancer, a study comparing clinical incidence with autopsy findings reported that the ratio of small papillary thyroid cancer incidence in women to men is 4.39:1, while the pooled odds ratio of subclinical papillary thyroid cancer at autopsy in women compared with men is 1.07, with the incidence rate ratio decreasing as cancer lethality increased and a thyroid cancer mortality ratio by sex of 1.02:1. That pattern indicates much of the apparent female excess in thyroid cancer reflects detection rather than underlying disease. On the secondary claim, the study the post cites by PMID and DOI is Brito et al 2021 in JAMA Internal Medicine. That paper is a cross-sectional study of national data for commercially insured and Medicare Advantage enrollees analyzing levothyroxine use over time, in which 110,842 patients newly initiated levothyroxine treatment between 2008 and 2018. A peer-reviewed editorial summarizing that literature states it is estimated that about 30% of persons newly started on l-thyroxine treatment in the United States have normal thyroid function, and a separate investigation of prescription appropriateness found 31% in alignment with American Thyroid Association guidelines while 54% of new prescriptions were considered suboptimal because thyroid function results were unconfirmed or normal. Contemporary coverage of the Brito paper reported that more than half of U.S. patients initiating levothyroxine therapy in a 10-year period failed to meet the threshold for benefit. The "21 million of 23 million" figure does not come from that study. It comes from Yale School of Medicine news coverage of a separate publication: as many as 90 percent of those who take levothyroxine may have been unnecessarily prescribed the medication, with approximately 23 million Americans taking levothyroxine, a claim attributed to Joe El-Khoury, PhD, associate professor of laboratory medicine, whose argument is that natural seasonal variation in thyroid hormones is not accounted for by current laboratory testing, which he warns is leading to an enormous number of unnecessary prescriptions, set out in a letter published in Clinical Chemistry on March 1.