Let's take a quick trip back in history to the late 1800s. It was a time when hypothyroidism was, for some patients, a fatal and untreatable condition. Those who survived still lived a life of fatigue, depression, weight gain, and dulled thinking and concentration.
That's why it was so groundbreaking when in 1981, Dr. George Redmayne Murray began using an extract from the thyroid glands of sheep to treat the most severe cases of hypothyroidism successfully. Dr. Murray's lifesaving discovery eventually evolved into natural desiccated thyroid (NDT) drugs. NDT, made from the desiccated thyroid glands of pigs, is still in use today as one treatment option for hypothyroidism. In the United States, the NDT brands available include Armour Thyroid, Nature-throid, WP Thyroid, and NP Thyroid.
Levothyroxine didn't come on the scene until the middle of the twentieth century. This synthetic form of the thyroid hormone thyroxine (T4) was touted as the modern treatment for hypothyroidism, casting NDT as old-fashioned and out-of-date. We currently have several levothyroxine brands available in the US, including Synthroid, Levoxyl, Unithroid, and Euthyrox tablets; several generic manufacturers of tablets; and Tirosint gel capsules and Tirosint-SOL oral solution.
NDT went from a peak of 16.6 million prescriptions in 1966 to only 4.5 million prescriptions in 1988, as levothyroxine became the standard, accepted treatment for an underactive thyroid. Since the late 1980s, NDT usage has grown again. According to a 2018 survey conducted by the American Thyroid Association, around 30 percent of hypothyroid patients currently take natural desiccated thyroid.
The choice between NDT and levothyroxine is, however, controversial. Some health care providers are willing to prescribe any thyroid hormone replacement drug that can safely and best resolve a patient's hypothyroidism. Others are adamant about only prescribing levothyroxine and prefer not to treat patients with NDT.
Ahead, a look at why some doctors don't support treatment with NDT.
Why won't some doctors support thyroid treatment with NDT?
The T4/T3 Issue
Your body needs two primary thyroid hormones made by your thyroid gland: T4 (thyroxine) and T3 (triiodothyronine). T4 is a storage hormone whose only role is to convert into T3—the active hormone used by cells and tissues in your body.
The typical daily T3 requirement in a 150-pound healthy adult is around 30 mcg. A healthy thyroid gland itself produces about 5 mcg of that T3. The remaining 25 mcg of T3 needs to be made by T4 to T3 conversion. Mainstream endocrinology believes that if you take adequate dosage of only synthetic T4 (levothyroxine), enough of that T4 will convert into T3 to meet your daily needs.
Since the introduction of levothyroxine, some patients have anecdotally reported much better symptom relief and management of their hypothyroidism by taking NDT vs. levothyroxine. The likely reason? NDT contains natural forms of both T4 and T3 hormones.
Experts are beginning to understand that—for reasons that include nutritional status, stress, and other hormonal imbalances—some patients don't convert T4 to T3 as well as thought. More recently, research has shown that a subset of the population has a genetic defect that impairs the ability to convert T4 into T3. Many of these patients report far better symptom relief and quality of life taking NDT because it includes T3 and doesn't rely solely on conversion.
Still, some doctors believe that measuring T3 levels and treating low-normal or low T3 is unnecessary or unbeneficial. They prefer not to treat patients with NDT drugs.
Potency and consistency
Decades ago, NDT's potency and consistency were standardized based on its iodine content, not its T4 or T3 content, resulting in some potency fluctuations. Unfortunately, some practitioners still believe that NDT isn't reliable or consistent.
However, the truth is that now NDT manufacturers must follow FDA guidelines on what's known as "Good Manufacturing Practice," which standardizes NDT based on the T4 and T3 content.
Specifically, the US Pharmacopeia (USP) monograph for thyroid tablets specifies that a 1 grain (60 or 65 mg) tablet must contain 38 mcg T4 and 9 mcg T3—with a margin of error of plus or minus 10%. The measured amount of T4 and T3 in a 1 grain NDT tablet must, therefore, be between 34.2-41.8 mcg of T4 and 8.1-9.9 mcg of T3.
NDT manufacturers are also required to ensure that their medications meet those potency standards throughout each lot's expiration date.
One reason given by some doctors for their reluctance to prescribe NDT is the fact that NDT drugs are not "FDA-approved." This statement is misleading. NDT was available by prescription long before the Food and Drug Administration came into existence. These drugs were "grandfathered in" by the FDA and were never required to go through the formal FDA approval process. They are, however, carefully regulated by the FDA. Companies manufacture NDT drugs according to FDA guidelines on "Good Manufacturing Practice," and their formulations follow strict standards set by the US Pharmacopeia (USP).
Myths and misinformation
You may also hear some popular—but misinformed—statements from some practitioners. For example, some doctors have publicly stated that "NDT is made from cows, and can cause mad cow disease." The truth is that all NDT drugs in the US are porcine (from pigs). There have been no cases of "mad cow disease" associated with NDT drugs.
You may also hear some healthcare providers claim that NDT does not require a prescription, but instead, you can buy it at vitamin or health food stores. This statement is also incorrect. There are various over-the-counter thyroid glandular supplements on the market, but they are not prescription NDT drugs. The four brands of NDT medication used to treat hypothyroidism all require a doctor's prescription.
Which thyroid medication is best for you?
For some patients, levothyroxine works well to resolve hypothyroidism. In other cases, you may feel best with the addition of T3, whether by adding synthetic T3 (liothyronine) to your levothyroxine treatment or taking NDT.
Suppose you take levothyroxine and need a better resolution of your symptoms. In that case, you may want to discuss with your thyroid doctor if natural desiccated thyroid might be a good option for you.
As a thyroid patient, you're probably asking the operative question: "Which thyroid medication is best for me?" There's no definitive answer. Ultimately, the best thyroid hormone replacement regimen for you is the medication, brand, and dosage that best and safely manages your hypothyroidism and relieves your symptoms. Finding out which medication and dosage will work best for you is often a trial-and-error process. It's essential to be informed about all of your options and work with a thyroid doctor who is open to all treatment options to determine if NDT drugs are right for you.