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The top US regulator of medicines this week said he wants to officially approve an animal-derived thyroid drug, seemingly addressing concerns sparked by a press release from his own agency.

On August 6, the FDA sent letters to manufacturers, importers, and distributors of animal-derived thyroid medications, stating that the agency “intends to take action against marketed unapproved animal-derived thyroid (ADT) products (sometimes described as desiccated thyroid extract [DTE] products).”

“Due to their complex biological origin, these medications contain many compounds that are uncharacterized for safety and effectiveness,” the FDA said, noting that it could take as long as “12 months to safely transition patients to an FDA-approved thyroid hormone replacement product.”

“FDA is not taking immediate action against manufacturers who make unapproved animal-derived thyroid medication to give patients time to transition to an FDA-approved medication to treat their hypothyroidism,” the agency said.

Following this announcement, in an August 13 post on his verified account on X, FDA Commissioner Marty Makary wrote that his agency “is committed to pursuing the first-ever approval of desiccated thyroid extract, pending results of the ongoing clinical trials” and “will in the meantime ensure access for all Americans,” without specifying what kind of thyroid medications would be available.

The X post came after considerable social media backlash to the FDA’s plans to shift patients toward synthetic thyroid medications and away from animal-derived versions.

FDA-approved laboratory-made, or synthetic, thyroid medications currently dominate the US market.

About 22 million people received prescriptions for levothyroxine through outpatient retail pharmacies in 2024 compared to an estimated 1.5 million patients who received prescriptions for animal-derived versions, the FDA said.

Marketed under names such as Armour Thyroid, NP Thyroid, Nature-Thyroid, and Natural Thyroid, these medications are produced from dried, ground animal thyroid glands, usually those of pigs.

Complex History

The FDA has had a long and challenging history with regulating thyroid medicines. These drugs were first sold in the US well before federal laws gave the FDA the regulatory powers it now has, Antonio Bianco, MD, PhD, of the University of Texas Medical Branch, Galveston, Texas, and former president of the American Thyroid Association, told Medscape Medical News.

“Because they existed before the FDA, their regulatory pathway is really complicated,” Bianco said.

In 1965, four of every five prescriptions for thyroid hormone were for natural thyroid preparations, as pharmacologic authorities confirmed that these were highly effective, well-absorbed, and produced clinically predictable outcomes, wrote Roselyn Cristelle Mateo-Collado, MD, of Rush University, Chicago, and James V. Hennessey, MD, of Harvard Medical School, Boston, in a 2019 review in Endocrine. But reports of patients not responding to desiccated thyroid triggered concerns about inconsistencies in the potency of these tablets, they added.

Over time, a newer synthetic version, levothyroxine, became more widely used, even with concerns arising as well about inconsistent potency.

Between 1987 and 1994, the FDA received 58 adverse experience reports associated with the potency of orally administered levothyroxine products. In 1997, the FDA mandated that any company wishing to market levothyroxine must first secure agency approval.

In 2002, Abbott Laboratories received approval to keep selling its levothyroxine drug, Synthroid, which had been on the market for more than 40 years and was being taken by about 8 million people in the US, Abbott said at the time.

In 2013, Abbott spun off its drug business as AbbVie. AbbVie now markets both Synthroid and Armour Thyroid, a desiccated thyroid drug.

What’s Next?

In his X post, the FDA chief Makary did not mention any specific studies that he expects to lead to approval of a desiccated thyroid drug.

AbbVie is sponsoring a phase 2/3 study examining the safety of its Armour Thyroid against the synthetic thyroid medicine, according to the ClinicalTrials.Gov website. AbbVie declined to comment on this story.

Other companies also are working on research studies for their animal-derived thyroid drugs, so the FDA’s August 7 announcement came as an unwelcome surprise for many people, Bianco said. Makary’s X post this week likely was meant to respond to pushback from supporters of the animal-derived drugs, he added, noting that the commissioner’s tone may signal that the agency is “correcting course” and minimizing the impact of the agency’s announcement.

“The manufacturers are working very hard” on large-scale trials, Bianco said. “They require money and time to be done properly, and I think that’s what the FDA should be stimulating.”

Bianco reported being a consultant for AbbVie, Acella, Aligos, and Synthonics.

Kerry Dooley Young is a freelance journalist based in Washington, DC. She has covered medical research and healthcare policy for more than 20 years.