Levothyroxine sodium (L-thyroxine) is the most popular thyroid medication. It is a preferred treatment for hypothyroidism due to known consistent potency, stability, easy measurement, and long half-life (~7 days).
While most doctors believe levothyroxine (Synthroid) alone is the best way to treat hypothyroidism, many people cannot respond to synthetic L-thyroxine (T4) and need an alternative.
Liothyronine sodium (Cytomel, Triostat) is a synthetic version of T3 (triiodothyronine). It is rarely used alone to treat hypothyroidism. Liothyronine may be preferred in myxedema coma due to its faster onset of action.
The problem with liothyronine is a short half-life (1 day), leading to fluctuations in T3 levels.
Natural thyroid hormone alternatives
Natural thyroid hormone replacement medications are made from desiccated (dried) pork thyroid glands. They contain both levothyroxine (T4) and liothyronine (T3). In addition, they also contain the thyroid hormones T1 and T2. Little is known about T1 and T2, but they do occur in the human body and may be one reason some people do well on these natural extracts.
Armour® Thyroid tablets provide 38 mcg of levothyroxine (T4) and 9 mcg of liothyronine (T3) per grain. Its main disadvantage is less predictable potency and stability.
As Armour® Thyroid provides both T4 and T3 thyroid hormones, instead of just providing T4, as levothyroxine do, it helps people when levothyroxine (Synthroid) does not.
Levothyroxine vs Armour Thyroid
There is a controversy over whether to use synthetic thyroid hormone like Synthroid versus natural forms (Armour Thyroid)2.
Armour Thyroid was the only treatment for hypothyroidism for about 50 years. It was found that the amounts of T3 and T4 varied greatly from batch to batch. Once a synthetic T4 was synthesized it has began to be widely used because it does not have similar problems of standardization.
The body converts thyroxine (T4) into triiodothyronine (T3), which is the hormone that the body uses at the cellular level. In some people, combination products may be better than single T4 products, because of a lack of ability to convert T4 to T3.
Recently, a synthetic T3/T4 combination (Liotrix) has been developed to deal with the standardization problems of Armour Thyroid. However, Liotrix (Thyrolar) is not available.
Equivalence of thyroid medications
Liothyronine (T3) is approximately 3-4 times as potent as levothyroxine (T4) on a microgram for microgram basis.
The basic rule in converting thyroid doses is that 100 micrograms of levothyroxine is roughly equivalent to 25 micrograms of liothyronine, or 1 grain (60mg) of Armour® Thyroid1. These doses produce equivalent normalization of TSH.
Substitutes for Synthroid
The main approved Levothyroxine products are:
The primary difference among these medications is that each brand has different ﬁllers, binders, and dosages. Some people may have allergies to certain brands, or have problems with pill size.
Knoll Pharmaceutical, the manufacturer of Synthroid, made misleading claims like “There is no Substitute for Synthroid", "FDA has not determined bioequivalence among levothyroxine sodium products", and "No proven bioequivalent product". However, Knoll failed to reveal facts to such representations.
In 1990s, Knoll delayed the publication of the study3 that demonstrated that less expensive brand-name Levoxyl and generic versions of Synthroid were equivalent to and could be substituted for Synthroid. The study was published in the April 16, 1997, issue of the Journal of the American Medical Association.
Author: OriginalDrugs Team