Natural desiccated thyroid: a fixed ratio, and a regulatory status worth knowing
Thyroid extract from pig glands, in clinical use since before the FDA existed — which turns out to be the key to its regulatory status. It supplies both hormones in a ratio you cannot change, and that constraint shapes everything about how it reads on a panel.
- Also known as
- NDT, Armour Thyroid, NatureThroid, WP Thyroid
- Class
- Thyroid hormone
- Regulatory status
- NOT FDA approved. Desiccated thyroid predates the 1938 legislation that created the modern approval pathway and has been marketed ever since under enforcement discretion rather than through an application — no manufacturer has submitted potency, purity or bioequivalence data for FDA review. Prescription required. Dispensed by most pharmacies and by compounding pharmacies.
- Storage
- Room temperature and dry, in the original container with whatever desiccant came in it, out of light.
- Once opened
- Unchanged once opened, as long as it stays dry.
- What ruins it
- Humidity is the real enemy — a bathroom cabinet is the worst place in the house for these.
- Handling caveat
- General handling practice for this formulation. Your supplier’s own insert or COA overrides anything here.
A fixed ratio you do not control
Established clinical use Desiccated thyroid is standardised by content and supplies both T4 and T3 in roughly a four-to-one ratio by weight. Human thyroid output is closer to fourteen to one. That difference is the central pharmacological fact about NDT: relative to what a human gland secretes, it is a T3-heavy preparation, and the consequences follow from that.
Because the ratio is fixed, dose is one dial rather than two. Raising the T4 raises the T3 proportionally, so someone who needs more of one and not the other cannot get there from here — which is exactly the situation combination therapy with separate levothyroxine and liothyronine exists to address. NDT is simpler and less adjustable; separate hormones are more work and fully titratable.
The T3 fraction also carries T3’s measurement problem. It has a day-long half-life against thyroxine’s week, so the level moves across the day, and a free T3 drawn without a recorded interval since the dose is hard to place. The thyroid panel page covers what each value means.
The approval question
Established clinical use Desiccated thyroid products are not FDA-approved drugs. They predate the 1938 legislation that created the modern approval pathway and have been marketed ever since under enforcement discretion rather than through an application. No manufacturer has taken one through approval, and the agency has periodically said so.
That matters more than a regulatory footnote usually does, because approval is what forces a manufacturer to demonstrate potency, purity and bioequivalence to a regulator before a product reaches a pharmacy. Nobody has done that for these. Unapproved is not the same as unavailable, though — these are ordinary prescription drugs dispensed by ordinary pharmacies, which is why the distinction gets lost.
The practical consequence is the one the app’s drawbacks list already names: potency consistency between batches and between brands is a real historical problem for these products, and there have been recalls over it. Switching brand or batch is a reason to recheck rather than assume equivalence — more so than with levothyroxine, where the same caution applies for a narrower reason.
Reading the panel
Off-label or community practice TSH suppresses on adequate NDT, as it does on any preparation containing T3, and that is expected rather than a finding. It also means the marker that titrates levothyroxine stops being the one to steer by, which is what the app’s own note says — guide by free T3 rather than TSH.
The argument about whether people do better on NDT than on levothyroxine is the same argument the T3 page covers, with the same unsettled evidence: randomised trials of combination therapy have mostly not shown consistent benefit on measured outcomes, and some blinded preference work has found patients choosing the T3-containing option. Neither finding closes it, and anyone presenting it as closed in either direction is overstating.
Established clinical use The risk that accrues quietly is the same too: sustained excess thyroid hormone reduces bone mineral density and raises atrial fibrillation incidence, neither of which announces itself. That is why resting heart rate sits on the app’s monitoring list beside the hormones, and why this is a prescribing decision rather than a self-titration. Anything on the drawbacks list below belongs with the clinician who prescribes.
The dosing rows the app records
Off-label or community practice Reproduced from the app’s reference so you can see what it holds, not as a recommendation. Which row applies to a particular person, if any, is a clinical decision this page does not make — and rows describing supraphysiological or post-cycle use are filtered out before this table is built, so what you see here is a subset.
| Label | Amount | Route and frequency | Duration recorded |
|---|---|---|---|
| Starting | 15-30mg/day (1/4 to 1/2 grain) | Once daily oral | 6-8 weeks then assess |
| Therapeutic | 60-120mg/day (1-2 grains) | Once or twice daily | Ongoing |
What the app monitors alongside it
The panel below is the app’s own monitoring note for Natural Desiccated Thyroid, verbatim. The analytes in it that have a page here are linked; those pages cover what each one measures, which assay produced it and how to read a trend.
- Monitoring panel
- Free T4, Free T3, TSH, heart rate. TSH typically suppresses — guide dose by Free T3 target.
Drawbacks and risks the app records
From the app’s own entry, and the potency-variation item is the one with a real history behind it rather than a theoretical one.
- Variable potency between batches and brands historically
- Porcine source — not suitable for some dietary restrictions
- T3 component can cause peaks and troughs
- Requires same careful titration as synthetic hormones
- TSH suppression expected
- Some insurers don't cover it
Interaction rules that name Natural Desiccated Thyroid
From the app’s interaction data, filtered so no rule naming a compound this site does not publish appears. Not exhaustive, and not a safety clearance: a combination that is not listed is one nobody has documented here, which is not the same as one that is fine. The combination checker has the rest.
NDT Already Contains T3 — Avoid Adding More Without Monitoring
NDT contains both T4 and T3 (approximately 4:1 ratio). Adding additional T3 can cause hyperthyroidism symptoms. If symptoms suggest inadequate T3, adjust NDT dose before adding separate T3.
What to watch: Free T3, Free T4, TSH, heart rate (target 60-80 resting). Symptoms of hyperthyroidism.
Questions people actually ask
Is NDT FDA approved?
No. These products predate the 1938 approval pathway and are marketed under enforcement discretion rather than through an application, so no manufacturer has had to demonstrate potency, purity or bioequivalence to a regulator before selling one. They are still prescription drugs dispensed by ordinary pharmacies — unapproved is not the same as unavailable.
What does "one grain" mean?
It is the historical unit for desiccated thyroid, roughly 60-65 mg of thyroid powder, standardised by hormone content. The app’s dosing rows use both the milligram and the grain figure, which is how these products are labelled.
Why is my TSH suppressed?
Because the preparation contains T3, which feeds back on the pituitary directly. That is expected on an adequate dose and is why free T3 rather than TSH is what the app’s own monitoring note steers by.
Is it better than levothyroxine?
The randomised evidence has not settled it. What is different is that NDT gives both hormones in a fixed ratio, which is simpler but not adjustable — where separate T4 and T3 are more work and fully titratable. Which suits a person is a prescribing conversation.
Where the load-bearing numbers come from
Named rather than linked. Publisher URLs move, and a citation that resolves to a 404 two years from now is worse than one you can search for by name — every entry below is findable from the title and year alone.
- T4 to T3 ratio in desiccated thyroid versus human secretion
- Standard endocrinology references; roughly 4:1 in the preparation against roughly 14:1 physiologically
- Regulatory status of desiccated thyroid products
- Marketed as pre-1938 products under enforcement discretion; no manufacturer has completed an approval
- Randomised evidence on T3-containing therapy
- Trials and meta-analyses from the 2000s onward, alongside blinded preference studies reporting a different result
- Bone density and atrial fibrillation with excess thyroid hormone
- Long established in the literature on subclinical hyperthyroidism
A preparation whose potency can vary between batches makes the brand and the date part of the result. TherapyLog records both.
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