Three different types of thyroid medications restore thyroid hormone levels in hypothyroid patients, and each has pros and cons.
Levothyroxine or synthetic T4 is the Standard of Care that endocrinologists have been taught to prescribe. Synthroid is the oldest brand name, and generics are also available for very little cost. Tirosint is a fairly new brand of T4 in gel cap form, with no food colors in any dose. The problem with taking only a T4 medication is that a normal thyroid gland secretes a little T3 in addition to T4.  So to truly replicate a normal thyroid gland, both T4 and T3 should be taken.  In fact, the presence of T3 itself helps T4 conversion to T3 during prolonged critical illness. 
Cytomel or liothyronine (synthetic T3) is available by prescription, but difficult to get prescribed. Most doctors have been taught that T4 is all that is necessary, so that’s all most will prescribe.
Natural desiccated thyroid (NDT) is available in the US as Armour Thyroid and Nature-throid, but has had an availability problem in the last couple of years. Many patients also reported that a reformulation of Armour in 2009 made the medication ineffective for them. NP Thyroid became available in Nov 2010, but patients reported availability problems in 2013. WP Thyroid became available in 2013. Erfa Thyroid, which is most like Armour before it was reformulated, can be ordered from Canadian pharmacies, but ample lead time is required to get through customs, and it may not be reimbursed by insurance. A doctor’s prescription is required to place an order. Shipment time from Canada has varied from four days to five weeks for me. The five-week delay did not appear to be a customs issue; it was likely a local problem because we have had other problems with other US packages that never arrived.
Which medication should one choose? Well like most things medical, that depends. There are some who are militant that desiccated thyroid is the only way to go. But it is contraindicated in some. A few people on Graves’ forums have reported that their TSI antibodies increased and their thyroid eye disease worsened after they started desiccated thyroid. The theory is that their antibodies recognized the pork antigens (desiccated thyroid is made from pig thyroids), which triggered an attack. Returning to synthetic T4 and/or T3 has often resolved the problem. That said, there are many with Graves’ who take desiccated thyroid successfully. So there are really no firm guidelines. If your antibodies (TPO, Tg, TRab) increased or decreased significantly after starting desiccated thyroid, please share your story on the blog post: Desiccated Thyroid contains T4, T3, and rT3, but little T2 or T1
T4 medications like levothyroxine usually result in lab results with a free and total T4 in the upper part of the range, but a free and total T3 in the lower half. When they report lingering hypothyroid symptoms, their doctors usually prescribe additional T4. Not surprisingly, this can cause the T4 to rise above the reference range. This does not match the lab result of a person with a healthy thyroid. To simulate a healthy thyroid’s output, one would have to take both T4 and T3. One study compared the total T3/T4 ratios of healthy people to the ratios of T4-medicated patients and found a lower ratio in the T4-medicated patients. A very simple way to equalize the ratios would be to add some T3. 
Most people on thyroid forums that have been on synthetic T4 and then switched to desiccated thyroid report feeling better, especially if they have no thyroid gland. So it’s a popular medication, it’s just difficult to find a doctor to prescribe it. Desiccated thyroid is made from whole pig thyroids and therefore contains all the components found in a thyroid gland: T4, T3, reverse T3 (rT3), diiodotyrosine (DIT), monoiodotyrosine (MIT), thyroglobulin, thyroperoxidase enzyme (TPO), hydrogen peroxide (H2O2), iodide, tyrosyl residues, and calcitonin (a hormone involved in calcium and bone metabolism).  Calcitonin is not a thyroid hormone, but another hormone produced by the parafollicular cells, which just happen to be located within the thyroid gland. Prescription calcitonin is administered by nasal spray or injection, because it is not absorbed well orally, so its effect in desiccated thyroid may be minimal. However, there is probably a synergy with all these additional components that make desiccated thyroid a more complete and effective replacement therapy for some than the synthetic replacements. People on desiccated thyroid tend to have healthier bones, but that may be from the T3, not the calcitonin. Studies show that T3 is essential to bone health.
T2 and T1 are supposedly the special ingredients in desiccated thyroid, but thyroid physiology textbooks say that only T4, T3, MIT, and DIT are produced in the thyroid gland, not T2 and T1. Only trace amounts of T2 have ever been measured in thyroglobulin in the thyroid , and T1 cannot be formed by the available iodotyrosine building blocks, MIT (1 iodine) and DIT (2 iodine). When a pair of iodotyrosines (MIT and DIT) couple together, they form a thyroid hormone, also known as an iodothyronine. MIT + DIT cannot form T1, because 1 + 1 cannot = 1. Perhaps the confusion is because one of the precursors is called diiodotyrosine (DIT), while T2 is also known as diiodothyronine. The words are similar but they are two different things: DIT is a precursor with only one tyrosine ring, while T2 has two tyrosine rings and has measurable effects as a thyroid hormone. Diiodotyrosine (DIT), like T4 and T3, has been found in the thyroid and in serum. 
T2 and T1 are made from deiodination outside of the thyroid gland.  Deiodination is the process where an enzyme strips off one iodine from T4, resulting in T3 (or rT3). The process of deiodination continues and another iodine is stripped off forming T2 (there are three types of T2), then T1 (there are two types of T1), and eventually T0, otherwise known as a non-iodinated thyronine. 
T2 has been detected in both serum and thyroglobulin, but one study found the production rate of T2 was very close to the combined T3 + rT3 production rate. In other words, the T2 came from deiodination, not the thyroid gland.  Desiccated thyroid cannot therefore be considered a major source of T2, since T2 levels in the thyroid gland are negligible.
Thyroid hormone production was found to be pH dependent in one laboratory experiment. The higher the pH, the more hormone was formed. The majority of the hormone produced was T4, followed by rT3, T3, and then negligible amounts of T2. No T2 was produced if the pH was below 6, though the other hormones were still produced at a pH of 5. Another study referenced in this report determined that thyroglobulin from actual thyroid glands contained approximately 90% T4, 7.4% T3, 2.2% rT3, and 0.4% T2. 
If rT3 is made in the thyroid gland, then it would also be a component of desiccated thyroid. Anyone taking desiccated thyroid is ingesting some amount of rT3, which is a normal, benign metabolite of thyroid conversion, as discussed in the rT3 section. They should not be surprised if their rT3 levels rise as their desiccated thyroid dose rises.
Does T2 have any biological activity? Studies show that it does. In one study, rats were fed a high fat diet, and some rats were given T2 for 30 days, while the control rats were not. The T2 group showed reduced fat accumulation, triglycerides, cholesterol, fat content of the liver, and body weight, when compared to the control rats fed the same high fat diet, who were not given T2.  Those with impaired T4 to T3 conversion would also have impaired conversion to T2, because the same enzyme performs both types of conversion. T3 and rT3 are the precursors to T2, and desiccated thyroid provides both forms, so the essential raw material is more available to someone taking this type of medication than T4 alone. Anecdotally, this may explain why some people are unable to lose weight while on T4, but then lose the weight when they switch to desiccated thyroid. Interestingly, enzymes convert T2 from both T3 and rT3, but it is rapidly converted from rT3, and only slowly from T3. 
Many on desiccated thyroid have lab results that show a low in range Total and Free T4 when their Free T3 gets into the optimal range (upper half). This is because desiccated thyroid (pig thyroid) is approximately 80% T4, and 20% T3, while a normal human’s thyroid output is closer to 94% T4 and 6% T3.  The simple remedy for people who test low on T4 is to just add some synthetic T4 to their desiccated thyroid. Studies show that both T4 and T3 need to be optimal for mental and physical well-being. The brain and body are in two different compartments, and some organs prefer T4, others T3. 
Other people do not convert their T4 to T3 as readily, so their labs will look fine on desiccated thyroid, with both Total T4 and Free T3 in the optimal upper half of the range. Really poor converters will show Total T4 in the upper half, but Free T3 in the lower half. These people have underlying conversion problems that should be addressed. Are ferritin and cortisol optimal? Are there other drugs like beta blockers being taken that may interfere with conversion? Is the person diabetic? If all factors have been addressed, then the ideal dose might be desiccated thyroid + additional T3. Desiccated thyroid could be used as the base, with additional T4 or T3 added as necessary to bring the person’s thyroid labs into the optimal range, or to a point where they feel best. This is usually accomplished by trial and error.
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