What Does the FDA Letter to NDT Manufacturers Mean for You?

If you take thyroid medication, you may have seen recent headlines about the FDA’s new letter to makers of natural desiccated thyroid (NDT/DTE) products like Armour®, NP Thyroid®, and Nature-Throid®. Understandably, this has raised concerns for many thyroid patients who depend on these medications to control their hypothyroid symptoms.

So what exactly does this mean for you? Let’s break it down...

The FDA’s Recent Letter on NDT

On August 6, 2025, the FDA formally notified manufacturers, importers, and distributors that animal-derived thyroid products (NDT/DTE) are considered biological drugs. This means they now require an official Biologics License Application (BLA) to remain on the market.

Right now:

• No NDT manufacturer has this license.
• The FDA has allowed a 12-month transition period for patients and doctors to move toward FDA-approved therapies.
• The FDA says this step was taken because of safety and consistency concerns, including variable potency, contamination risk, and more than 500 adverse event reports collected over decades.

The good news? FDA leadership has also expressed interest in pursuing the first-ever approval of NDT if ongoing clinical trials demonstrate safety and effectiveness.

A Brief History of NDT

Natural desiccated thyroid has been around for over a century—long before synthetic thyroid hormones existed.

• 1890s–1950s: The very first thyroid treatments came from dried, powdered animal thyroid glands (what we now call NDT). For decades, this was the only option available—and it worked well enough that it became the standard treatment.
• 1960s: Synthetic levothyroxine (T4) was developed. It was hailed as a more stable, standardized option compared to NDT, which sometimes varied in potency.
• 1970s onward: As lab testing for TSH became widespread, levothyroxine monotherapy became the default treatment. Medical guidelines shifted away from NDT almost entirely.
• 1985: NDT medication became standardized based on its thyroid content, not its iodine content which made its dosing more consistent.
• Today: Many surveys have shown that thyroid patients greatly prefer NDT over levothyroxine, likely because NDT contains not just T4 but also T3, T2, T1, iodine and calcitonin. Despite this, NDT has remained technically “unapproved” by the FDA since it was grandfathered in before modern drug regulations. The August 2025 letter is the FDA’s latest power move to bring these products under modern approval standards.

FYI, other drugs that were “grandfathered” in because they were in use prior to 1938, when the Food, Drug, and Cosmetic Act (FDCA) first required proof of safety for new drugs:

• Aspirin
• Codeine
• Morphine
• Digoxin
• Nitroglycerin
• Phenobarbital
• Colchicine
• Hydrocortisone

Why Switching Everyone to Synthroid Doesn’t Work

Based on this letter, the FDA wants all thyroid patients to be on synthetic thyroid hormone, such as Synthroid (levothyroxine). In The New Thyroid Handbook, I explain why a “levothyroxine-for-all” approach falls short for many patients:

1. Levothyroxine is only T4.
   Synthroid provides T4—the storage form of thyroid hormone. The body must convert T4 into T3, the active hormone that drives energy, metabolism, mood, and cognition. Many patients struggle with this conversion and remain symptomatic despite normal TSH and free T4 levels.
2. Conversion problems are common.
   Some experts estimate that 14-20% of hypothyroid patients struggle with T4 to T3 conversion.

I personally think that is too low.

For example, conditions such as diabetes and insulin resistance have been shown to impair T4 to T3 conversion.

The CDC’s National Diabetes Statistics Report (2022 edition) found that 50% of the US population has either diabetes or prediabetes.

If half of us have diabetes and insulin resistance, it only makes sense that a lot of us will therefore have impaired T4 to T3 conversion.

When you add things such as stress, chronic illness, autoimmune conditions, nutrient deficiencies (selenium, zinc, iron, vitamin D), certain medications, aging, and even genetics, I personally think that most of us have at least some degree of impairment of T4 to T3 conversion.

3. Patients need individualized therapy, not a one-size-fits all approach.
   Some patients do fine on levothyroxine alone. Others do best with combination therapy (T4 + T3) or NDT, which provides the full spectrum of thyroid hormones (T4, T3, T2, T1, calcitonin). Optimal care means matching the treatment to the patient—not forcing every patient into the same box.

And Remember—Even Synthroid Has Problems

It’s easy to assume that FDA-approved levothyroxine is flawless. But that’s not true.

In just the last few months alone there have been multiple recalls of levothyroxine:

• April 2025: Accord Healthcare initiated a voluntary recall—later upgraded to a Class II recall—of several lots of levothyroxine sodium (25–175 mcg) due to subpotent assay results below the 95% potency threshold. Affected lots included D2300323, D2400547, D2300044, and D2400725, with expiry dates through March 
• June 20, 2025: A broader recall of 9 additional levothyroxine lots from Accord Healthcare was issued following stability testing failures—again for sub‑95% potency.
• July 2025: Over 160,000 bottles of levothyroxine sodium were recalled—a Class II recall upgrade—due to confirmed subpotent active ingredients. The recall covered multiple strengths (25–175 mcg) that failed potency specifications, with expiration dates through March 
• Late 2024 / Early 2025: Mylan had previously recalled numerous levothyroxine lots due to both superpotent and subpotent failures—with the FDA classifying the recall as precautionary and noting the risk to patients was negligible.

My point is this: no thyroid medication – synthetic or natural – is infallible.

Even prescriptions considered standard-of-care can vary batch to batch, potentially affecting how well the medication works for you.

This highlights the importance of ongoing monitoring of you and your thyroid condition.

It also reinforces the fact that every patient is different. Trying to treat all thyroid patients with the same medication would not work and would be a disservice to a lot of people.

What This Means for You

If you’re currently taking NDT:

• Don’t panic. You do not have to stop today. The FDA has allowed a year for all of this to take effect.
• The Alliance for Pharmacy Compounding (APC), representing compounding pharmacies, issued a statement noting they are currently “actively evaluating the letter and considering appropriate legal strategy.” They’ve committed to keeping members updated as the situation evolves.
• In response to strong patient and industry feedback, the FDA Commissioner posted on X (formerly Twitter) that while the agency expects compliance, they are committed to pursuing the first-ever approval of desiccated thyroid extract, contingent on ongoing trial results. He also emphasized that access for patients will continue “in the meantime.” 
• Industry sources say manufacturers are working diligently on large-scale trials, understanding these will require significant time and investment.
• There is a strong possibility that either the FDA will back off of their threats, or possibly some ongoing trials by the NDT manufacturers will result in FDA approval of NDT medication.
• In the unlikely event that the FDA shuts down production of NDT in 2026, you still have some options.

• There are available synthetic combinations of levothyroxine and liothyronine that may work for many patients.
• Liothyronine can be prescribed as a separate medication as well as levothyroxine.
• Compounding pharmacies can make levothyroxine with sustained-release T3 (expensive but effective).
• None of these are ideal, but at least we can do something.

Bottom Line

The FDA’s letter is about regulatory status, not about erasing your experience. Hopefully a solution will be found over the next year. Either the FDA will back off, or one or more of the NDT-producing companies will be able to submit long term safety studies that will receive FDA approval.

If nothing changes, there are alternative options available. Talk to your medical provider.

If you want to take a deeper dive into this and other thyroid topics, take a look at my book, The New Thyroid Handbook, or take my Advanced Thyroid Management online course.

Blessings to each of you,

Jeff Whelchel, MD