Armour Thyroid & NP Thyroid: What the American Thyroid Association Guidelines Say

T3 vs. T4: The Two Main Thyroid Hormones

The thyroid gland produces two active hormones: thyroxine (T4) and triiodothyronine (T3). T4 is the predominant form secreted by the thyroid — approximately 80% of thyroid output is T4. T3 is the more biologically active hormone; it directly enters cells and drives metabolic processes including heart rate, energy metabolism, body temperature, mood, and cognitive function.

The thyroid gland produces a small amount of T3 directly (~20%), while the majority is produced by peripheral conversion of T4 to T3 in tissues such as the liver, kidneys, and skeletal muscle. This conversion is carried out by deiodinase enzymes, which are selenium-dependent.

Standard hypothyroidism treatment with levothyroxine (synthetic T4) relies on this peripheral conversion pathway to supply the body's T3 needs. For most patients, this works well. However, some individuals have impaired conversion — meaning their T4 levels may be adequate but T3 remains low.

Total T3 Testing: ATA Guidance

The ATA guidelines state that TSH (thyroid-stimulating hormone) is the primary test for assessing thyroid function in most clinical situations. Free T4 is used as a secondary test. Routine T3 testing is not recommended for most patients with hypothyroidism.

However, Total T3 (which measures both bound and free T3 in the bloodstream) is appropriate in specific situations:

• Diagnosing hyperthyroidism — Total T3 is often the first test to become elevated in Graves' disease and toxic thyroid nodules, sometimes before TSH is suppressed
• Evaluating T3 toxicosis — a condition where T3 is disproportionately elevated relative to T4
• Monitoring patients on combination T4/T3 therapy or desiccated thyroid extract
• Assessing poor T4-to-T3 conversion in select patients with persistent symptoms on levothyroxine
• Thyroid cancer monitoring — in select cases during levothyroxine suppression therapy

Normal Total T3 Range

Reference ranges vary by laboratory. A typical Total T3 range is approximately 80–200 ng/dL (or 1.2–3.1 nmol/L). Values should always be interpreted in the context of your full thyroid panel and clinical symptoms.

T4 to T3 Conversion: When It May Be Impaired

A subset of patients taking levothyroxine continue to feel unwell despite a normal TSH and free T4. The ATA acknowledges this as a recognized clinical challenge, though the mechanism is not fully understood. Factors that may impair T4-to-T3 conversion include:

• Selenium deficiency (deiodinase enzymes are selenium-dependent)
• Chronic illness, major surgery, or prolonged physiological stress
• Liver or kidney disease (major sites of peripheral conversion)
• Genetic variants in deiodinase genes (DIO2 polymorphism) — some individuals are genetically less efficient converters
• Caloric restriction or prolonged fasting

When conversion is impaired, standard labs (TSH, free T4) can appear normal while peripheral T3 availability remains suboptimal — creating a gap between test results and how the patient actually feels.

Combination T4/T3 Therapy: What the ATA Says

The ATA guidelines (2014) do not recommend combination T4/T3 therapy as routine first-line treatment for hypothyroidism. The majority of patients are well-managed on levothyroxine alone. However, the guidelines acknowledge that:

• A subset of patients do not feel well on levothyroxine monotherapy despite normal TSH
• In carefully selected patients, a trial of combination levothyroxine + liothyronine (T3) may be considered, with shared decision-making between patient and physician
• Combination therapy requires careful monitoring of TSH and Total T3 to avoid overtreatment
• Liothyronine (Cytomel) has a shorter half-life (~24 hours) than levothyroxine, which can cause T3 level fluctuations and occasional symptoms of excess (palpitations, anxiety)

Desiccated Thyroid Extract: Armour Thyroid & NP Thyroid

Desiccated thyroid extract (DTE) — sold under brand names such as Armour Thyroid and NP Thyroid — is derived from porcine (pig) thyroid glands and contains both T4 and T3 in a fixed ratio (approximately 4:1 T4 to T3 by weight). It has been used to treat hypothyroidism for over a century.

ATA Position on Desiccated Thyroid

The ATA guidelines note that DTE is not routinely recommended as first-line therapy, primarily because:

• The fixed T4:T3 ratio may not match individual patient needs
• The T3 content can cause supraphysiologic T3 peaks, which may not be well-tolerated in all patients
• Long-term data on outcomes compared to levothyroxine is limited

However, the ATA also acknowledges that some patients prefer DTE and report feeling better on it compared to levothyroxine alone. The guidelines state that a trial of DTE may be considered in patients who continue to have hypothyroid symptoms on levothyroxine, as part of shared decision-making.

Our Providers' Experience with Armour Thyroid and NP Thyroid

At District Endocrine, Dr. Anis Rehman has extensive clinical experience prescribing and managing patients on Armour Thyroid and NP Thyroid as treatment options for hypothyroidism. For carefully selected patients — particularly those who have not responded well to levothyroxine — DTE can be a meaningful alternative or complementary approach. Monitoring includes regular TSH and Total T3 levels to ensure therapeutic balance and avoid overtreatment.

Persistent Symptoms on Levothyroxine

If you are taking levothyroxine and your TSH is within the target range but you continue to feel unwell, the ATA recommends:

1. Confirming optimal TSH target — not just "in range" but at the level where you feel well (typically 0.5–2.5 mIU/L for most patients)
2. Reviewing absorption — levothyroxine should be taken on an empty stomach; many supplements, medications, and foods interfere with absorption
3. Checking Total T3 — to assess whether peripheral conversion is adequate
4. Investigating other causes of fatigue, weight gain, or cognitive symptoms (vitamin D deficiency, anemia, depression, sleep apnea)
5. Discussing a trial of combination therapy or DTE if conversion appears impaired and other causes have been excluded