NDT vs SR-T3 vs Cytomel vs Levothyroxine: The Complete 2026 Thyroid Medication Comparison

For anyone exploring thyroid hormone research beyond standard levothyroxine monotherapy, the field opens into four primary medication categories: levothyroxine (T4), immediate-release T3 (Cytomel), sustained-release T3 (SR-T3), and natural desiccated thyroid (NDT). Each has a different composition, a different pharmacokinetic profile, a different regulatory status, a different cost structure, and a different patient pattern it fits best.

This guide is the complete 2026 comparison - hormone content, dosing equivalence, pharmacokinetics, side effects, FDA approval status, sourcing, and a decision framework for which formulation fits which research subject.

Research framing. This article reviews thyroid hormone pharmacology and the clinical literature comparing formulations. It is not medical advice. T3-containing products on this site are sold as research reference standards and are not approved for human consumption. See our research-use-only disclaimer.

The Four Formulations at a Glance

Composition: What's Actually in Each

Levothyroxine (T4)

Synthetic L-thyroxine sodium - chemically identical to endogenous T4. Sold as Synthroid, Levoxyl, Tirosint, and generic levothyroxine. Pure T4, no T3. The body must convert T4 to T3 via deiodinase enzymes (DIO1, DIO2) for the hormone to be metabolically active.

Cytomel / Liothyronine (immediate-release T3)

Synthetic L-triiodothyronine sodium - chemically identical to endogenous T3. Sold as Cytomel and generic liothyronine. Pure T3, no T4. Bioactive immediately on absorption; no conversion required.

Sustained Release T3 (SR-T3)

Synthetic L-triiodothyronine sodium identical to Cytomel's active ingredient, but formulated in a hydroxypropyl methylcellulose (HPMC) slow-release matrix that extends drug release over 4-8 hours instead of 30 minutes. SR-T3 is only available through compounding pharmacies or as a research-grade reference standard - no FDA-approved sustained-release liothyronine product exists.

Natural Desiccated Thyroid (NDT)

Powdered, desiccated pig thyroid gland. Contains T4, T3, T2, T1, and calcitonin in roughly the proportions found in porcine thyroid tissue. Standardized per grain (~60-65 mg) to approximately 38 mcg T4 + 9 mcg T3, with smaller amounts of T1 and T2 whose clinical significance is debated. Sold as Armour Thyroid, NP Thyroid, WP Thyroid, Nature-Throid (historically), and compounded NDT.

The presence of T1 and T2 is one of NDT's distinctive features - these are largely absent from synthetic formulations. Their biological roles are less understood than T3 and T4, but some research suggests T2 has direct mitochondrial effects independent of conversion.

Pharmacokinetics: The Most Important Difference

The single biggest functional difference between these formulations is how T3 reaches the bloodstream over time.

The functional consequences:

• Levothyroxine works well for patients whose conversion is intact. It fails for patients with impaired DIO1/DIO2 activity, high reverse T3, chronic illness, or genetic variants in deiodinase enzymes.
• Cytomel produces direct T3 effect but the spike is the dose-limiting factor - palpitations, anxiety, sleep disruption appear before the patient can reach a sustained therapeutic effect.
• SR-T3 produces direct T3 effect without the spike - the same total dose of T3 is more tolerable when delivered over 4-8 hours.
• NDT combines the sustained T4 backbone with a moderate T3 spike. For patients who tolerate it, NDT is often the most physiological feeling formulation; for those who don't tolerate the T3 spike, NDT can be too stimulating.

Dose Equivalents (Most-Cited Clinical Ratios)

The most commonly cited dose-equivalence ratios:

Our T3 conversion calculator handles these conversions automatically across all four formulations.

Note that potency-equivalent ≠ pharmacokinetic-equivalent. Switching from 100 mcg T4 to 25 mcg Cytomel produces a very different serum profile even though total T3 receptor exposure is comparable. SR-T3 is the formulation that most cleanly preserves both potency AND a physiological-looking curve.

Side Effect Profiles Side-by-Side

Cytomel's peak-driven side-effect profile is the main reason patients eventually switch to either NDT or SR-T3.

FDA Status and Sourcing

A common source of confusion: the FDA never formally approved Armour Thyroid through a New Drug Application - it has been grandfathered into the US market since before modern FDA approval requirements existed. NP Thyroid was approved through a more formal process. Both are legal, available with prescription, and widely used.

The Decision Framework: Which Formulation for Which Pattern

"Standard" primary hypothyroidism with intact conversion

T4 monotherapy (levothyroxine) is the most evidence-supported first-line approach. Cheap, FDA-approved, smooth pharmacokinetics. Most patients do well on it.

Persistent symptoms on T4 monotherapy despite normal TSH

Combination T4 + SR-T3 (or T4 + low-dose Cytomel) following ETA guidelines. Typical: maintain T4 at 75-125 mcg/day plus add 5-15 mcg SR-T3 twice daily. Target T4:T3 ratio of 13:1 to 16:1 by weight.

Reverse T3 dominance (FT3:rT3 ratio < 10)

Wilson's WT3 protocol with SR-T3 monotherapy. This is the protocol most associated with clearing rT3 dominance - 7.5 mcg SR-T3 every 12 hours, titrated up by 7.5 mcg per dose every 1-3 days until body temperature sustains at 98.6°F for 3 consecutive weeks. Use our FT3:rT3 calculator to assess your ratio first.

"Just doesn't tolerate Cytomel" but needs T3 supplementation

Switch from Cytomel to SR-T3. Same active molecule, much smoother pharmacokinetics. Most patients who fail Cytomel due to palpitations or anxiety tolerate equivalent SR-T3 doses without issue.

Patient prefers a "natural" feeling formulation

NDT (Armour Thyroid or NP Thyroid) provides the T4 + T3 combination plus T1, T2, and calcitonin in physiological proportions. The Hoang et al. 2013 crossover trial showed 49% of patients preferred NDT over levothyroxine when given both blinded.

MCAS / multi-allergen / chronic illness sensitivity

Clean SR-T3 with HPMC + MCC only. Standard compounded SR-T3 often contains dyes, lactose, PEG, or other allergen-class excipients. The patient population most likely to need SR-T3 is also the population most likely to react to those additions. Our SRT3 line uses HPMC + MCC only - no dyes, no allergen fillers.

Subclinical hypothyroidism (elevated TSH, normal free T4)

Low-dose T4 (25-50 mcg/day) is the standard approach. Combination therapy is not first-line for subclinical patterns.

Where the Evidence Is Strong vs Weak

To stay honest about what's well-established:

• Strong: Levothyroxine effectively treats most primary hypothyroidism. Cytomel and SR-T3 both contain bioactive T3 that produces predictable serum effects. NDT is biochemically active and produces measurable thyroid hormone effects.
• Moderate: ETA guidelines support combination T4 + T3 trials for symptomatic patients on T4 monotherapy with normal TSH. SR-T3 is preferred over Cytomel for tolerability based on pharmacokinetic reasoning and clinical experience.
• Hypothesis-supported: Specific protocols (Wilson's WT3, the SR-T3 monotherapy approach) are reasoned from solid pharmacology but lack large randomized trials. Reverse T3 dominance as a formal clinical entity is not recognized by mainstream endocrinology (though the biochemistry is uncontested).
• Patient preference: NDT vs levothyroxine preference data (Hoang 2013) is real but the sample is small and the mechanism unclear.

Related Tools and Reading

• T3 dosing & conversion calculator - convert between all four formulations
• FT3:rT3 ratio calculator - assess your reverse-T3 dominance
• Sustained Release T3 (SR-T3): The Complete 2026 Guide
• Reverse T3: The Complete 2026 Guide
• The Wilson's T3 Protocol (WT3)

Frequently Asked Questions

What is the difference between NDT, SR-T3, Cytomel, and levothyroxine?

Levothyroxine is pure synthetic T4. Cytomel is pure synthetic T3 in immediate-release form. SR-T3 is the same synthetic T3 in a slow-release matrix that spreads delivery over 4-8 hours. NDT (Armour, NP Thyroid) is desiccated pig thyroid containing T4 + T3 + T1 + T2 + calcitonin in roughly natural proportions, standardized to ~38 mcg T4 + 9 mcg T3 per grain. They differ in active hormones, pharmacokinetic curves, FDA status, and which patient patterns they best fit.

Is NDT better than Cytomel?

For most patients - yes, in the sense that NDT contains both T4 and T3 plus T1, T2, and calcitonin, providing a more complete physiological profile. Cytomel is pure T3 with no T4 backbone, so it's best used as an add-on to existing T4 therapy rather than a standalone replacement. The exception is patients with severe reverse-T3 dominance, where pure T3 (SR-T3 in particular) is preferred over NDT specifically to avoid the T4 substrate driving more rT3 production.

Is SR-T3 better than NDT?

For patients with significant rT3 dominance, autoimmune sensitivities, or pork allergy, yes - SR-T3 avoids the T4 backbone (no rT3 substrate) and avoids the porcine source. For patients tolerant of NDT who need both T4 and T3 supplementation in physiological proportions, NDT is often more convenient (one tablet, predictable composition). SR-T3 is more flexible because it can be combined with separate T4 dosing to fine-tune the T4:T3 ratio.

What is the T4 to T3 conversion ratio?

The most commonly cited clinical conversion ratio is 1 mcg T3 ≈ 4 mcg T4 by potency. So 100 mcg of levothyroxine roughly equals 25 mcg of T3 (Cytomel or SR-T3) in total receptor activation. For NDT, 1 grain (~65 mg) ≈ 38 mcg T4 + 9 mcg T3 ≈ 74 mcg T4-equivalent. Use our T3 conversion calculator for automatic equivalents.

Why don't most doctors prescribe SR-T3?

Three reasons. First, no FDA-approved sustained-release liothyronine product exists - SR-T3 must come from a compounding pharmacy, which not all doctors are familiar with. Second, mainstream endocrinology guidelines still consider levothyroxine monotherapy first-line for most hypothyroidism, with combination therapy reserved for select cases. Third, physician familiarity: most endocrinologists trained on Cytomel for T3 supplementation when needed, not SR-T3 - the SR formulation is more common in functional and integrative medicine practices.

Can I take NDT and SR-T3 together?

Yes - some patients use NDT for its T4 backbone and add small amounts of SR-T3 for additional T3 coverage when NDT alone is insufficient. The challenge is dose tracking: NDT already contains both T4 and T3, so adding SR-T3 requires careful attention to total T3 dose. Most combinations of this kind would reduce NDT to 0.5-1 grain and add 5-10 mcg SR-T3 twice daily, monitoring labs at 6 weeks.

Which formulation is best for Hashimoto's thyroiditis?

It depends on the stage. Early Hashimoto's with normal labs: typically watchful monitoring with thyroid-supportive nutrients (selenium 200 mcg/day, vitamin D, zinc). Established hypothyroidism from Hashimoto's: typically starts with levothyroxine; combination T4 + SR-T3 added if symptoms persist on T4-only despite normal TSH. Patients with high rT3 from chronic Hashimoto's inflammation: the Wilson's WT3 protocol with SR-T3 monotherapy is often considered.

What's the cost difference?

Roughly, monthly cost in the US (2026 ballpark): levothyroxine generic $4-15; Cytomel/generic liothyronine $30-80; NDT (Armour or NP) $40-90; compounded NDT $80-160; compounded SR-T3 $80-180 depending on strength and pharmacy. Research-grade reference standards have separate pricing.

Closing Note

The four major thyroid formulations occupy different niches in the research landscape. Levothyroxine remains first-line for most primary hypothyroidism with intact conversion. Cytomel is convenient but peak-limited. NDT combines T4 + T3 in physiological proportions and is well-loved by patients who tolerate it. SR-T3 is the formulation of choice for patients who need direct T3 supplementation without Cytomel's peak side effects, want flexible T3 dosing without NDT's fixed T4:T3 ratio, or need a clean excipient profile compatible with MCAS and multi-allergen sensitivities.

For research-grade SR-T3 reference standards in HPMC + MCC formulation only, see our SRT3 catalog. All compounds discussed here are sold strictly for laboratory study and are not approved for human consumption.