Benefits
Thyroid support claims for glandular supplements are untested in people
No controlled human trial has tested whether an over-the-counter thyroid glandular supports thyroid function. The only randomized trial of pig thyroid used a prescription drug in people already diagnosed and treated. Any effect a glandular capsule has may come from leftover hormone: in one laboratory analysis, 9 of 10 thyroid support supplements contained T3 and 5 of 10 contained T4.
Combined T4 and T3 delivery (prescription drug only)
Prescription desiccated thyroid supplies both T4 and T3 in a fixed ratio. In a randomized crossover trial it did not improve symptoms or neurocognitive measures compared with levothyroxine. American Thyroid Association guidelines keep levothyroxine as the standard of care and found no consistent, high-quality evidence that thyroid extract works better. This does not apply to over-the-counter glandulars.
Patient preference for the prescription form in one trial
In a randomized crossover trial of 70 adults treated for low thyroid function, 48.6% preferred prescription desiccated thyroid, 18.6% preferred levothyroxine and 32.9% had no preference, although symptom and neurocognitive scores did not differ. This describes a clinician-dosed prescription drug, not a dietary supplement benefit.
Mechanism of action
Preformed thyroid hormone
Prescription desiccated thyroid, and over-the-counter products that contain leftover hormone, act by supplying ready-made T4 and T3, which bind thyroid hormone receptors in cells and raise metabolic rate, the same way pharmaceutical thyroid hormone works.
Fixed T4 to T3 content from animal tissue
The Armour Thyroid label lists 38 mcg T4 and 9 mcg T3 per grain, about four parts T4 to one part T3 by weight. Levothyroxine, the standard treatment, supplies T4 alone, which the body converts to T3 as needed.
Glandular "tissue therapy" idea (unsupported)
Marketing of hormone-free glandulars rests on the idea that eating animal thyroid tissue nourishes the human thyroid. No published human evidence shows that oral glandular tissue improves thyroid function once the hormone is removed.
Clinical trials
Randomized, double-blind crossover trial; prescription desiccated thyroid or levothyroxine for 16 weeks each (Hoang et al. 2013, J Clin Endocrinol Metab).
70 adults aged 18 to 65 with primary hypothyroidism on a stable levothyroxine dose for 6 months.
Symptoms and neurocognitive measures did not differ between the two treatments, and quality of life did not improve significantly. Weight was about 3 lb lower on desiccated thyroid, an effect of thyroid hormone dosing rather than a weight-loss use. At the end, 48.6% preferred desiccated thyroid, 18.6% preferred levothyroxine and 32.9% had no preference. This tested a prescription drug, not an over-the-counter glandular.
Laboratory analysis by high-performance liquid chromatography of 10 commercially available thyroid support supplements (Kang et al. 2013, Thyroid).
Over-the-counter thyroid support products (not patients).
9 of 10 products contained T3 (1.3 to 25.4 mcg per tablet) and 5 of 10 contained T4 (5.77 to 22.9 mcg per tablet). At the recommended dose, 5 products supplied more than 10 mcg of T3 a day and 4 supplied 8.57 to 91.6 mcg of T4 a day, some above common treatment doses.