Benefits
Thyroid hormone replacement (prescription form only)
Standardized prescription porcine desiccated thyroid supplies both T4 and T3 and can treat diagnosed hypothyroidism under medical care. This is a regulated drug effect, not a property of over-the-counter glandular supplements. Prescription desiccated thyroid is prescribed, dosed and adjusted by a clinician using blood tests; no over-the-counter glandular is a substitute for it.
Combined T4 + T3 delivery
Because pig thyroid contains T3 as well as T4, desiccated thyroid delivers both hormones in a fixed ratio. Some patients report feeling better on it, though the randomized crossover trial did not show it outperforms standard levothyroxine on neurocognitive or quality-of-life outcomes. This describes the FDA-regulated prescription drug only: American Thyroid Association guidelines do not recommend desiccated thyroid extract over levothyroxine as routine first-line therapy, and no over-the-counter glandular supplement has been shown to deliver this effect.
Patient preference in some individuals
In a randomized crossover trial of treated hypothyroid patients, a substantial minority preferred desiccated thyroid over levothyroxine, but it did not improve cognition or quality-of-life endpoints. Preference is not the same as a superior outcome. This finding applies to a prescription medication dosed and monitored by clinicians in diagnosed hypothyroid patients; it is not a dietary-supplement benefit and does not transfer to over-the-counter glandular capsules.
Marketed for general "thyroid support" (unproven)
OTC glandulars are promoted for energy, metabolism, and thyroid support. There is no controlled evidence that hormone-free glandular tissue improves thyroid function, and any perceived benefit may come from undisclosed hormone contamination. In one laboratory analysis, 9 of 10 over-the-counter thyroid health supplements contained measurable T4 and/or T3, some at amounts large enough to alter the user's thyroid function.
Mechanism of action
Preformed thyroid hormone
Prescription (and hormone-contaminated OTC) desiccated thyroid acts by supplying preformed T4 and T3, which bind nuclear thyroid hormone receptors and regulate metabolic rate, the same mechanism as pharmaceutical thyroid hormone.
Fixed T4:T3 ratio from animal tissue
Pig thyroid provides a higher proportion of T3 relative to T4 than the human thyroid secretes, which can produce supraphysiologic T3 peaks after dosing compared with T4-only levothyroxine.
Glandular "tissue therapy" theory (unsupported)
Marketing of hormone-depleted glandulars rests on an unproven idea that consuming animal thyroid tissue nourishes the human thyroid. No credible evidence supports oral glandular tissue improving thyroid function once hormone is removed.
Clinical trials
Randomized, double-blind, crossover trial; 70 hypothyroid patients; 16 weeks per arm (Hoang 2013).
Adults with primary hypothyroidism on stable hormone replacement.
In this trial the prescription drug was well tolerated and was associated with modest weight loss — an effect of thyroid hormone dosing, not a weight-loss indication — and about 49% of patients preferred it to levothyroxine. However, it did not improve neurocognitive function or most quality-of-life measures versus levothyroxine. Applies to the standardized prescription drug, not OTC glandulars.
Laboratory analysis of commercially available thyroid health supplements (Kang 2013).
Over-the-counter thyroid support products (not patients).
9 of 10 tested supplements contained detectable triiodothyronine (T3) and/or thyroxine (T4), some at clinically relevant amounts, despite being sold as dietary supplements that should be hormone-free. Demonstrates the contamination risk of glandular products.