Armour Thyroid vs Synthroid/Levothyroxine

Question:
Dr. Tan, can you explain the difference between Armour Thyroid and Synthroid or Levothyroxine? Do you have a preference between the two?

Answer:

If you've been diagnosed with hypothyroidism, you'll know that Armour Thyroid, Synthroid, Levothyroxine are all different types of thyroid hormone medication.

First, some background. The thyroid gland produces different types of thyroid hormone, the most important of which are T4 and T3.  T4 makes up the majority of thyroid hormone made by our thyroid glands, it is weaker and is considered to be a "reserve" form of the hormone. T4 is converted into T3 (which is considered to be more potent or active) in peripheral tissue such as the liver, kidney and intestines.   Nature is very wise to do this--because it makes sense for the body to produce more of a weaker hormone and have it in "reserve" and then later on have it converted in to the more active type of hormone later on, when needed.


Synthroid, levothyroxine, levothroid are all different medication names for T4. These are the most widely prescribed type of thyroid medication, usually by endocrinologists. The assumption is that the person taking these medications are able to convert T4 (reserve form) to T3 (active form) efficiently.  The benefits of taking plain T4 is that it is stable, it has a long half life, and patients would often not feel a major difference if they do not take their dose at exactly the same time everyday.

Armour Thyroid is made from dried pig thyroid gland and therefore contains both T4 and T3 naturally. The ratio of T4 and T3 in Armour Thyroid is similar to human thyroid hormone output, but not exactly the same. The benefit of taking Armour Thyroid is that it naturally contains T3 (active form) already and may be helpful for people who do not convert T4 to T3 well (there may be several reasons for this including nutritional deficiencies).  The downside is that if a patient skips a dose, they will feel more than if they were taking plain T4 because the T3 component of Armour Thyroid is more potent and it also has a shorter half-life in the body.  Nature-Throid, West-Throid are other brand names of T4/T3 combination thyroid medications.

Do I have a preference?  Each option--whether Synthroid (T4) vs. Armour Thyroid (which is a combintaion of T4 and T3)--have their strengths and weaknesses.  Synthroid has the benefit of being more stable (because it lasts longer in the body, and is therefore less resistant to ups and downs or missing a dose or missing the timing of the dose).  Synthroid is also the more familiar type of thyroid medication among family doctors and endocrinologists and it is considered not controversial.  Its weakness is that some people are not able to efficiently convert T4 to T3, so even if they have enough Synthroid (T4), they can still have hypothyroid symptoms.  Overall, in my practice, I would say that most people do pretty well on Synthroid.

Armour Thyroid's main strength on the other hand stems from the fact that its T4 and T3 ratio more closely resembles how our body naturally produces thyroid hormone--which is a combination of both T4 and T3.  Its main weakness is that because it is naturally derived, there is some deviation in terms of hormone levels in different batches of medication.   For example, a few years ago, Armour Thyroid was temporarily unavailable because it had to correct this problem.

Also, some patients may have problem taking a medication sourced from pigs/animals;  in these situations, if a patient is NOT doing well on Synthroid/Levothyroxine/Levothroid despite normal TSH levels, and need some T3, I may add in some Cytomel which is a pharmaceutical form of T3 to augment the T4.

Update, 4/24/2012:

In the last six months, I have noticed that my patients are not getting the same type of results from Armour Thyroid.  This also happened a few years ago when Armour Thyroid was briefly out-of-stock because of some issues with standardizing the amounts of T4 and T3--because Armour Thyroid is from a natural source, there is some variability in terms of the amount of T4 and T3 in every batch.  Not all patients seem to notice this, but the more sensitive ones do.

Lately,   I have been switching my most sensitive patients to a combination of Levothyroxine (T4) with an additional dose of Cytomel (T3--the more active type of hormone).   The ratio I start with approximates the amount of T4 and T3 that a healthy person would make naturally.   Cytomel is available as regular prescription (from Walgreens or CVS for example) or as a compounded time-release form (so that it lasts longer in the body, and you avoid the "peak then valley" associated with medicines that have a shorter half-life).