The cornerstone of hypothyroidism treatment — T4, T3, and NDT: how each works, clinical dosing, monitoring, and how to find a provider who actually tests Free T3.
Levothyroxine (T4) and liothyronine (T3) are the two active thyroid hormones used in hormone replacement. T4 is the long-lasting storage form; the body converts it to active T3 as needed. Natural desiccated thyroid (NDT) — Armour Thyroid, NP Thyroid — provides both, plus T2 and T1, from porcine thyroid glands.
All are prescription-only, FDA approved for hypothyroidism. Standard of care is T4-only replacement (Synthroid, levothyroxine). Patients who feel poorly on T4 alone may do better with combination T4+T3 or NDT — which typically requires finding a provider comfortable prescribing beyond the basic protocol.
FDA approved. Prescription only. Not WADA prohibited at replacement doses, but elevated exogenous thyroid use flagged in some sport contexts.
Levothyroxine (T4) is biologically inactive until deiodinated to T3 in peripheral tissues. Long half-life (~7 days) makes once-daily dosing feasible and stable.
Liothyronine (T3) is the biologically active hormone that binds nuclear thyroid receptors. Short half-life (~1 day) means dose fluctuations are felt more sharply. Some patients have impaired T4→T3 conversion (DIO2 polymorphisms) and do better with direct T3.
Desiccated porcine thyroid contains T4, T3, T2, T1 in a fixed ratio (~80:20 T4:T3). Natural ratio differs from human thyroid; some patients feel better on it, others don’t. Works well when baseline T4-only treatment leaves lingering symptoms.
| Benefit | Evidence |
|---|---|
| Hypothyroid symptom resolution | Gold-standard replacement; typically normalizes within 6–12 weeks of proper dosing |
| Energy & metabolism | Restored BMR and energy in hypothyroid patients |
| Cognitive function | Brain fog resolves with euthyroid restoration |
| Weight management | Resolves weight gain caused by hypothyroidism; not effective as a weight-loss drug in euthyroid patients |
| T4+T3 combo vs T4 alone | Trials do not show an average benefit. The ATA guideline (PMID 25266247) concluded levothyroxine "should remain the standard of care" with no consistently strong evidence for combination therapy or thyroid extract, and a JCEM meta-analysis reached the same verdict (PMID 16670166). The subgroup hypothesis is genetic — see below |
Why some patients still do better on T3. Panicker 2009 (PMID 19190113) found carriers of the rarer CC genotype at DIO2 rs225014 — about 16% of the population — had worse psychological well-being on T4 alone and improved more on T4/T3 combination, by 2.3 GHQ points at three months. The polymorphism did not change circulating thyroid hormone levels, which is why labs can look fine while a patient does not feel fine.
Two honest caveats. The authors state plainly that the result requires replication, and it has not been convincingly replicated since. And a genotype present in 16% of everyone is not a personalised explanation for any individual. This is a real hypothesis with real mechanistic logic, not an established finding — which is a better reason to trial combination therapy under a provider than a claimed 10–20% response rate that no trial produced.
Build your protocol, log every dose, monitor your body's response, and get reminders so you never miss a dose.
Start Tracking FreeNot medical advice. These are prescription medications. The figures below describe established clinical practice and labelled prescribing information — they are not a recommendation, they do not account for your labs, history, or other medications, and safe use depends on monitoring a page cannot do. Work with a qualified healthcare provider.
TSH remains the primary monitoring test in every major guideline, and for straightforward T4 replacement it is usually sufficient on its own. A fuller panel is genuinely useful when symptoms persist despite a normal TSH, when T3 or NDT is in use (both distort the TSH–symptom relationship), or at baseline for autoimmune screening:
Long-term over-replacement is the real hazard here, and it is well quantified. In the Framingham cohort (Sawin 1994, PMID 7935681), adults over 60 with TSH at or below 0.1 mU/L had a 10-year atrial fibrillation incidence of 28% versus 11% with normal TSH — a threefold adjusted relative risk (3.1, 95% CI 1.7–5.5). A 2024 umbrella review across 75,898 patients (PMID 38581565) put fracture risk in subclinical hyperthyroidism at RR 1.36 (95% CI 1.18–1.56). Read that number precisely: it covers subclinical hyperthyroidism of any cause. The estimate the same review reports for L-thyroxine suppression therapy specifically — the situation described here — is higher at RR 1.98, but its confidence interval crosses 1 (0.98–3.98, p = 0.06), so the fracture signal from over-replacement is suggestive rather than established. "Optimizing" thyroid by running labs above range is not a free win — it buys a measurable rise in stroke-causing arrhythmia and broken bones.
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Thyroid Hormones (T3 / T4) is a prescription medication. StackTrax does not sell, prescribe, or facilitate purchase of prescription drugs.
Find a clinician who can order baseline lab work, screen for contraindications, monitor your response, and adjust dosing over time. Options to consider:
Before starting, you’ll typically want:
Avoid sources that offer prescription medications without labs, medical history, or licensed-provider oversight. If a telehealth service promises a prescription after a 5-minute questionnaire, that’s a red flag.
Build your protocol, log every dose, monitor your body's response, and get reminders so you never miss a dose.
Start Tracking FreeT4 (levothyroxine) is the long-lasting pro-hormone — biologically inactive until the body deiodinates it to T3 in peripheral tissues. Its ~7-day half-life makes once-daily dosing stable. T3 (liothyronine) is the active hormone that actually binds nuclear thyroid receptors, with a much shorter ~1-day half-life so dose fluctuations are felt more sharply. Some patients have impaired T4-to-T3 conversion (DIO2 polymorphisms) and do better with direct T3.
Both are FDA approved for hypothyroidism. T4-only (Synthroid, levothyroxine) is the standard of care and works well for most patients. NDT (Armour, NP Thyroid) is desiccated porcine thyroid containing T4, T3, T2, and T1 in a fixed ~80:20 T4:T3 ratio. The natural ratio differs from human thyroid; some patients feel better on NDT, others don't. Mixed trials suggest about 10–20% of patients feel substantially better on combination therapy than T4 alone.
T3 (or NDT) is typically considered when baseline T4-only treatment leaves lingering symptoms despite normalized labs. Patients with impaired T4-to-T3 conversion (DIO2 polymorphisms) are the clearest candidates. Adding T3 requires a provider comfortable prescribing beyond the basic T4 protocol — typical T3 starting dose is 5–10 mcg/day split twice daily, titrated 5–10 mcg every 2–4 weeks.
For straightforward T4 replacement, TSH is the primary monitoring test in every major guideline and is usually enough on its own. A fuller panel — Free T4, Free T3, and thyroid antibodies at baseline — earns its place when symptoms persist despite a normal TSH, or when T3 or NDT is in use, since both distort the TSH-to-symptom relationship. We would skip reverse T3: a review of 402,386 thyroid orders (PMID 29756541) found little evidence supporting it, and 0.1% of providers placed nearly 30% of all rT3 orders.
It resolves the weight gain caused by hypothyroidism — but it is not effective as a weight-loss drug in euthyroid (normally functioning) patients. Long-term over-replacement with suppressed TSH carries real cost: a threefold higher risk of atrial fibrillation in adults over 60 (28% vs 11% at ten years, PMID 7935681) and a likely rise in fracture risk (RR 1.36, 95% CI 1.18-1.56 for subclinical hyperthyroidism of any cause; the estimate specific to L-thyroxine suppression is higher at RR 1.98 but does not reach significance, 95% CI 0.98-3.98 — PMID 38581565). Optimizing thyroid by running labs above range is not a free win.
Take it in the morning, 30–60 minutes before food, calcium, or iron. Standard starting dose is 25–50 mcg/day in elderly or cardiac patients, or 1.6 mcg/kg for full replacement. Titrate by 12.5–25 mcg every 6–8 weeks based on labs. Symptoms typically resolve within 6–12 weeks of proper dosing.
Elevated heart rate or palpitations, anxiety, irritability, tremor, heat intolerance, sweating, insomnia, unintended weight loss, and diarrhea. Long-term over-replacement (suppressed TSH) increases risk of atrial fibrillation and bone density loss, so these signs should prompt a dose reassessment rather than being pushed through.
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StackTrax guides cover peptides and compounds that are not FDA-approved for the uses discussed. The dosing, reconstitution, and safety information is compiled from published research and community protocols for educational purposes only.
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