What is Armour Thyroid and How is it Used?
Armour Thyroid is a desiccated (dried) porcine thyroid extract containing both T4 and T3 hormones, but it is NOT recommended as first-line therapy for hypothyroidism due to lack of standardization, unpredictable T3 content causing supranormal T3 spikes, and absence of formal FDA oversight for consistency. 1, 2, 3
Composition and Pharmacology
Armour Thyroid is derived from pig thyroid glands and contains both thyroxine (T4) and triiodothyronine (T3) in a fixed ratio of approximately 4:1 3. Unlike synthetic levothyroxine, desiccated thyroid products contain the full complement of thyroid hormones as they exist naturally in animal thyroid tissue 1.
The key pharmacologic problem is that serum T3 frequently rises to supranormal values during the absorption phase after ingestion, which can cause palpitations and other symptoms of transient hyperthyroidism 1. This occurs because T3 has a much shorter half-life than T4 and is absorbed rapidly, creating peaks that would not occur with endogenous thyroid hormone production 1.
Critical Safety Concerns
The most serious issue with desiccated thyroid products is the lack of standardization and FDA oversight. The T3 content varies between batches, and consistency is monitored only by manufacturers without formal regulatory oversight 2, 3. "Natural" thyroid preparations sold in health food stores may or may not contain active thyroid hormone, and their hormonal content is completely unknown 4.
This lack of standardization can lead to either:
- Relapse of hypothyroidism if the product is underdosed 4
- Iatrogenic hyperthyroidism if the product contains excessive hormone 4
One case report documented a 32-year-old male who developed an acute myocardial infarction while self-administering 120 mg of Armour Thyroid daily, with laboratory findings showing TSH 0.20 mIU/mL (low) and free T3 4.08 pg/mL (high), consistent with exogenous thyroid hormone excess 2. Hyperthyroidism increases cardiovascular event risk 2-3 times through propagation of a hypercoagulable, hypofibrinolytic state 2.
Why Levothyroxine is Preferred
Synthetic levothyroxine (L-T4) is the standard of care for hypothyroidism because it has multiple advantages over desiccated thyroid: 1
- Once-daily dosing due to long half-life (approximately 7 days) 1
- Forgiving of missed doses - occasional missing of a tablet causes no harm 1
- Preserves physiologic T3 production - extrathyroidal conversion of T4 to T3 normally provides 80% of daily T3 production and remains fully operative with L-T4 therapy 1
- Not associated with excess mortality with long-term use when properly dosed 1
- Predictable dosing - mean T4 dose required to normalize TSH is 1.6 mcg/kg per day 1
The American College of Clinical Oncology recommends levothyroxine as first-line therapy for all newly diagnosed hypothyroid patients 5.
Limited Role for Desiccated Thyroid
There may be a small subset of hypothyroid patients who, despite biochemical euthyroidism on levothyroxine, continue to complain of tiredness, lack of energy, discrete cognitive disorders, and mood disturbances 1. For these patients specifically, the American Thyroid Association guidelines suggest that combination therapy with LT4+LT3 can be considered on a trial basis 5, 3.
If desiccated thyroid extract is used, the mean daily dose needed to normalize serum TSH contains approximately 11 mcg T3, though some patients may require higher doses 3. However, this should only be considered after a trial of standard levothyroxine therapy has unambiguously failed to provide benefit 3.
Monitoring Requirements
For patients taking desiccated thyroid products, the same monitoring applies as for levothyroxine:
- TSH and free T4 should be checked every 6-8 weeks during dose titration 5
- Once stable, annual monitoring is appropriate 5
- Target TSH should remain in the reference range (0.5-4.5 mIU/L) to avoid risks of TSH suppression 5, 1
TSH values ≤0.1 mU/L carry significant risk of atrial fibrillation and bone loss, so TSH should not be allowed to fall below 0.2 mU/L 1.
Critical Pitfalls to Avoid
- Never recommend desiccated thyroid as first-line therapy - levothyroxine is the evidence-based standard 5, 3
- Warn patients against self-administration of "natural" thyroid products from health food stores, which have completely unpredictable hormone content 2, 4
- Do not use desiccated thyroid in patients with cardiac disease due to risk of T3-induced tachycardia and increased cardiovascular events 2
- Avoid desiccated thyroid in pregnancy - levothyroxine is the only appropriate therapy, as dose requirements increase 25-50% and precise dosing is critical for fetal neurodevelopment 6