MD, FACE
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Hyperthyroidism
Thyroid cancer
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Hyperthyroidism
Hypothyroidism
Nodular thyroid disease
Thyroid cancer
Most patients receiving thyroid hormone have hypothyroidism and are not able to manufacture normal levels of thyroid hormone on their own. The goal of thyroid hormone therapy is to create levels of thyroid hormone in the body which mimic the levels of thyroid hormone that would found in a person with a normal thyroid gland. Ultimately this translates into patients that no longer have symptoms of hypothyroidism and also have normal thyroid hormone levels in their blood.
Synthetic levothyroxine (T4) is biologically identical (bioidentical) to the primary thyroid hormone made by a healthy thyroid gland. It is produced my many manufacturers under a variety of Brand-names (Synthroid™, Levoxyl™, Unithroid™ and Levothroid™) as well as generic levothyroxine. We generally recommend staying on the same brand and not switching from one manufacturer to another. Although all brand of levothyroxine contain the same synthetic T4, there are different inert additives in the various products which can result in meaningful differences in the absorption of the various products. This will often impact the levels of thyroid hormone measured in the blood of the patient and can translate into how the patient feels on identical doses of one brand versus another. If it changing manufacturers is unavoidable (or you can’t afford a non-generic product), let your endocrinologist know when you change products so your dose can be appropriately monitored and adjusted.
Thyroid hormone can also be extracted from the thyroid glands of animals (manly pigs), dried, powdered and formulated into tablets or capsules. The most popular version of this product is marketed at Armour™. Thyroid hormone obtained from animals is sometimes incorrectly referred to as ‘natural’ thyroid. It is more correctly referred to as dessicated thyroid (DT). It has been heavily marketed as a ‘natural’ thyroid replacement hormone since it is not synthetically created in a lab. This is more of a marketing ploy, than anything else, since DT does contain a number of chemicals binders which hold the pill together as is seen with synthetic T4.
Prior to the discovery of individual thyroid hormones and our ability to synthetically manufacture them, DT was the main thyroid hormone used to treat hypothyroid patients. It has fallen out of favor and is now generally viewed as inferior to synthetic T4 for a number of reasons. To begin with, there is no scientific evidence that DT offers any advantage over synthetic T4. It is also important to note that the process of creating DT results in significant variation in the amount of T3 and T4 in each batch of DT. This can result in fluctuations in the blood levels of thyroid hormone after each new batch of medication is obtained from the pharmacy. Additionally, since DT obtained from animals is not purified, it often contains contaminating proteins that are not necessary for thyroid hormone replacement. DT therefore contains a mixture of T3 and T4 as well as a number of proteins that never exist in the body outside of the thyroid gland. Finally, the ratios of T3 to T4 found in DT—although perfect for a pig—are different from what normally exists in healthy humans. DT therapy therefore often results in levels of T3 that are higher than is normally found in humans, and levels of T4 that are too low. The end result is a product that is much more difficult to work with, and often leaves patients over-medicated.
T3 represents the biologically active version of thyroid hormone. It is created inside cells that use thyroid hormone by the removal of an iodine molecule from T4, which is a biologically inactive. A synthetic version of T3 is available (Cytomel™). Cytomel is not an appropriate treatment for hypothyroid patients for many reasons. First, the life span of T3 in the blood is very short. Patients taking T3 as a replacement hormone would need to be dosed many times a day. There are no FDA-approved long acting T3 products available on the market today, and ‘slow release’ T3 products that are sometimes prepared by compounding pharmacies should be viewed with great skepticism since they are not rigorously tested. In our experience they are no different in their biological activity than Cytomel. The blood levels of thyroid hormone levels that result from a T3-based replacement program are very unstable and never closely mimic what is seen in patients with normal thyroid function. T3 peaks very rapidly after dosing and falls precipitously a short time later. High T3 levels after dosing may cause palpitations, tremors, anxiety and insomnia followed by a ‘crash’ in energy levels associated with the subsequent drop in T3 levels. On the other hand, T4 has a very long life span in the blood and results in more balanced thyroid hormone levels across the 24 hour dosing interval—just like that seen in patients with normal thyroid function. Since all patients have the ability to convert T4 into T3, T4-treated patients are able to convert T4 to T3 as the body needs it. This protects the heart, bones and other organs from the toxic effects of excessive T3 levels that are seen with T3-based replacement programs.
Although T3 is made in very small amounts in humans with healthy thyroid glands, it is generally not used added to thyroid hormone replacement regimens. To begin with, scientific studies have shown no benefit in patients treated with a combination of T4 and T3 versus patients treated with T4 alone. Additionally, T4 and T3 combination therapy (such as Synthroid™ and Cytomel™) doubles the cost of therapy, since 2 co pays are required at the pharmacy. When you consider that a patient may take thyroid hormone for 40 or 50 years, this extra out-of-pocket expense can be very significant. Use of DT to avoid the extra co pay is an option, but still doesn’t get around the limitations outline above associated with DT itself.
There are many testimonials (especially on the internet) of patients that advocate the use of combinations of T4 and T3 for patients that don’t feel well on synthetic T4 alone. Several factors need to be considered in such patients before simply starting combination therapy (see Is It My Thyroid? for a more detailed discussion of this). Once this has been done, it may be worth a 3-6 month trial of T4/T3 combination therapy in exceptional cases.
