Desiccated thyroid or thyroid extract is produced by removing the thyroid glands of animals with similar anatomy and physiology to our own (most often pigs) which are then dried, powdered and manufactured into tablets for therapeutic use. Thyroid extracts are used to treat an underactive thyroid, technically known as hypothyroidism.
Thyroid extracts have been used since the late 1800s and are still used today, although their use has largely been replaced by synthetic thyroxine and T3 medications. Thyroid extracts may also be referred to as "natural thyroid", "natural thyroid hormones", thyroid USP, or by brand names including "Armour Thyroid", "Naturethroid", and "Westhroid".
Hypothyroidism in modern times is typically treated using mono-therapy with thyroxine (T4) based on blood test results for thyroid-stimulating hormone (TSH) and T4 levels. Although this brings satisfactory improvement for a large number of patients it does not work for all.
Some doctors argue that thyroid extracts are more effective because they contain both T4 and T3 thyroid hormones in roughly the same proportions as the healthy human thyroid produces itself. T3 is much more active in the body than T4 and some patients may have trouble converting T4 to T3 which could be one reason why some people respond better to thyroid extract than thyroxine medication. A small but significant number of patients also report that thyroid extract is more effective than thyroxine treatment.
Some of the thyroid extracts also contain T1 and T2 thyroid hormones. T1 is thought to be completely inactive but some evidence suggests T2 may play a role in stimulating TSH production and may stimulate the break down of fat for energy. Neither are included in conventional thyroid medications and treatments.
The counter-argument to the use of thyroid extracts is that the amounts of thyroid hormone they contain can vary quite significantly. Synthetic T4 and T3 medications on the other hand are carefully controlled for dosages. Simultaneous use of both T4 and T3 medication may therefore be a viable alternative to thyroid extract.
A couple of good books to learn about thyroid extract and all of these issues are:
Your Thyroid and How to Keep it Healthy - by Dr. Barry Durrant-Peatfield
Hypothyroidism Type 2: The Epidemic - by Dr. Mark Starr