Desiccated thyroid

Desiccated thyroid (also called natural thyroid, thyroid extract, or by the historical brand Armour Thyroid) is porcine thyroid gland, defatted and powdered, containing both thyroxine (T4) and triiodothyronine (T3) in a fixed ratio of roughly 4:1 by weight as required by the United States Pharmacopeia. It was introduced in 1892 by George Murray and has been continuously prescribed since, predating both the isolation of thyroxine (1914) and the synthetic levothyroxine of the 1950s. Modern endocrinology guidelines prefer synthetic Levothyroxine as first-line replacement, but a residual population of patients and clinicians prefer desiccated thyroid for its T3 content and historical track record.

Thyroid hormone, Hormone replacement

Thyroid (desiccated)

Armour Thyroid, NP Thyroid, Nature-Throid, WP Thyroid

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Summary

Classes

Thyroid hormone, Hormone replacement

Common uses

Hypothyroidism▲0▼, Goiter▲0▼

Pharmacy

Starting dose

30 mg PO daily (1/2 grain); titrate by TSH at 6-8 weeks; 60 mg desiccated thyroid is approximately equivalent to 88-100 mcg levothyroxine

Preparations

15, 30, 60, 90, 120, 180, 240, 300 mg tablets (1/4 to 5 grains; 1 grain = 60 mg)

US FDA Max

No fixed maximum; titrated to TSH target

Pharmacology

Routes

Oral

Onset

TSH normalization 4-8 weeks

Duration

Steady-state at 4-6 weeks

Half-life

T4 ~7 days; T3 ~1 day

Bioavailability

Variable; reduced by food, calcium, iron, PPIs

Pregnancy

Synthetic levothyroxine is the standard-of-care in pregnancy; desiccated thyroid use in pregnancy is not well studied^{[citation needed]}

Legal status

Rx-only in US

Purported mechanism

Desiccated thyroid supplies both T4 and T3 in a roughly 4:1 ratio; T4 is peripherally deiodinated to T3, but exogenous T3 supplementation produces faster onset and supraphysiologic post-dose peaks that synthetic T4 monotherapy avoids.▲0▼ Brand-to-brand and lot-to-lot variability in T3:T4 ratio is greater than with synthetic levothyroxine, which is why endocrine guidelines prefer the synthetic.