Natural desiccated thyroid (NDT) is a prescription medication made from the dried thyroid glands of pigs. It contains both major thyroid hormones your body naturally produces: T4 (the storage form) and T3 (the active form). NDT has been used to treat hypothyroidism, or underactive thyroid, for well over a century, making it one of the oldest hormone replacement therapies still in use.
What NDT Contains
Each tablet of NDT is measured in “grains,” an old apothecary unit. One grain (roughly 60 to 65 milligrams of dried thyroid powder, depending on the brand) delivers approximately 38 micrograms of T4 and 9 micrograms of T3. That T4-to-T3 ratio of about 4:1 is notably different from the human thyroid gland, which produces T4 and T3 at closer to a 14:1 ratio. This means NDT provides proportionally more T3 than your body would make on its own.
Beyond T4 and T3, porcine thyroid tissue contains small amounts of other compounds found in the gland, including calcitonin and trace thyroid proteins. Whether these additional components have any meaningful clinical effect remains unclear, but their presence is part of what distinguishes NDT from synthetic alternatives.
How It Differs From Synthetic Thyroid Medication
The standard treatment for hypothyroidism is levothyroxine, a synthetic version of T4 alone. The idea behind levothyroxine is straightforward: give the body T4, and it will convert what it needs into the active T3 hormone on its own. For most people, this works well. But a subset of patients report persistent symptoms like fatigue, brain fog, and weight gain even when their lab numbers look normal on levothyroxine.
NDT takes a different approach by supplying both T4 and T3 directly. Because T3 is the more potent and fast-acting hormone, NDT can produce noticeable effects more quickly after each dose. This is also what makes dosing trickier. T3 levels in the blood tend to spike in the hours after taking NDT, then drop off, which can cause fluctuations that don’t occur with levothyroxine alone.
What Clinical Trials Show
A well-known randomized, double-blind crossover study compared NDT directly to levothyroxine in patients with hypothyroidism. On standard measurements of symptoms, quality of life, and cognitive function, there were no significant differences between the two therapies. NDT did not produce a clear, measurable improvement over levothyroxine for the group as a whole.
The more interesting finding was in patient preference. Nearly half of participants (48.6%) preferred NDT over levothyroxine. Among those who preferred it, there were measurable benefits: they lost an average of four pounds during the NDT phase, and their scores on both a general health questionnaire and a thyroid symptom questionnaire improved significantly. This split, where objective group-level data shows no difference but individual patients clearly do better on one therapy, is part of what makes NDT controversial in endocrinology.
Risks and Side Effects
The main safety concern with NDT is its T3 content. T3 is roughly four times more potent than T4, and because it acts quickly, too much can push the body into a state of excess thyroid hormone (thyrotoxicosis). Symptoms of too much T3 include a racing heart, anxiety, tremor, shortness of breath, and agitation. In severe cases, excess thyroid hormone can cause heart rhythm problems and even damage to heart muscle.
These risks are not unique to NDT. Any thyroid hormone can cause problems at excessive doses. But NDT’s T3 component means there is less room for error, and the consequences of dosing mistakes can appear faster. Case reports have documented severe thyrotoxicosis from compounding errors where patients received dramatically higher T3 doses than intended, sometimes 10 to 1,000 times the prescribed amount. While compounding errors are rare, they highlight why consistent product quality matters.
At appropriate doses, the day-to-day side effect profile of NDT is similar to other thyroid medications. The most common issues relate to over-replacement: feeling jittery, difficulty sleeping, or a faster-than-normal heart rate. These typically resolve with a dose adjustment.
Current Brand Names
Several NDT products have been available in the United States, including Armour Thyroid, NP Thyroid, Nature-Thyroid, and Natural Thyroid. Armour Thyroid is probably the most widely recognized. In Canada and parts of Europe, Thyroid Erfa is a common option, with a slightly different formulation (35 mcg T4 and 8 mcg T3 per grain instead of the 38/9 split in most U.S. brands).
FDA Regulatory Status
NDT occupies an unusual regulatory position. Despite being sold in the U.S. for decades, these products have never gone through the FDA’s formal approval process. They were historically marketed under a kind of grandfather status because they predated modern drug approval requirements. The FDA has now reclassified animal-derived thyroid products as biological products, meaning they legally require an approved biologics license to be sold.
In August 2025, the FDA sent letters to manufacturers, importers, and distributors of unapproved NDT products, warning that they could face enforcement action for continuing to sell these medications without approval. The agency stated it is unaware of any studies that adequately demonstrate the safety and effectiveness of NDT products to the standard required for approval. This does not necessarily mean NDT is unsafe or ineffective. It means these specific commercial products have not been evaluated through the rigorous process the FDA now requires for biologics. For patients currently taking NDT, this regulatory shift could affect availability and may prompt conversations with prescribers about alternatives or transitions.
Who Uses NDT and Why
NDT is primarily prescribed for people with hypothyroidism who haven’t felt well on levothyroxine alone. Some clinicians also consider it for patients who have difficulty converting T4 to T3, though testing for this is imperfect. The typical patient seeking NDT has already tried standard therapy and wants a combination of both hormones rather than T4 by itself.
Patient advocacy communities have long championed NDT, and some people report dramatic improvements in energy, mood, and mental clarity after switching. These experiences are real but hard to study systematically, partly because individual responses to thyroid hormone vary enormously based on genetics, gut absorption, and other factors. The clinical trial data so far suggests NDT works about as well as levothyroxine on average, with a meaningful subset of patients doing noticeably better on it. Identifying who will benefit most before starting treatment remains an open question.