The thyroid hormone, thyroxine (T4) is a tyrosine-based hormone
which is produced by the thyroid gland. Synthetic versions of this
hormone are used to make up for a lack of naturally produced thyroxine
and increase the rate of metabolism. Artificially made thyroid hormone
improves the symptoms of hypothyroidism (underactive thyroid) and
speeds the rate of metabolism in those who suffer from the disease.
Thyroid hormones may also be used to treat goiter (enlarged thyroid
gland) and certain types of thyroid cancer.
In the early 1900's doctors found that injecting patients with thyroid
extracts from sheep reversed the symptoms of hypothyroidism. T4 was
synthesized in 1926. Thyroid drugs are only available with a
physician's prescription and are sold in tablet form. The most commonly
used T4 thyroid hormone is levothyroxine which is sold under the brand
names: Synthroid, Levoxyl, and Levothroid.
Thyroid hormone controls the rate of metabolism. When the thyroid is
under active, all body processes slow down and symptoms such as weight
gain, fatigue, and decreased body temperature are experienced. Through
supplementation of thyroid hormones, basal metabolic rate will be
Thyroid hormones are essential to proper development of all cells in
body. These hormones allow for the body to become more sensitive to all
other hormones, in turn making them more effective. Thyroid hormones
also regulate macronutrient (protein, fat and carbohydrate) metabolism,
therefore increasing protein synthesis and ultimately energy. This
allows for the body to burn more calories and use them more
sufficiently. For this reason, thyroid hormones are commonly used as
This medicine does not typically cause side effects as long as proper
dosages are administered. However some drawbacks of Thyroid drug use
are cardiac stress and possible loss of lean body mass. Negative
feedback in the thyroid can decrease natural production of thyroid
hormone, causing short term decrease of metabolic rate after use is
Many studies have proven that while calories and carbohydrates are
restricted, levels of diodinase, the enzyme that activates thyroid
hormone production, decreased. The conversion of T4 into T3, the active
form of thyroid hormone, was became hindered. Users discovered T3 to be
a better choice over T4 when using Thyroid drugs during Cyclic
T4 must be used in much higher doses to be as effective as T3. Subjects
found they needed to use a dosage of 300 mcg/day to achieve the same
results as 25-100 mcg/day of T3 (Cytomel).
Thyroid medication is frequently used by bodybuilders who are getting
ready for a contest, and really, I can't remember seeing a bodybuilder
dieting for any major show in the last decade who hasn't told me that
they use Thyroid medication. Most of the time, T4 is only used when T3
can not be found.
Recently though, due mostly to an article written by myself and
Dr.James Daemon, T4 has been experiencing a bit of a renaissance. This
is due to it being a bit more muscle sparing, and synergy with growth
hormone. Most bodybuilders use use GH for precontest dieting, and T4
has found a bit of a niche for those using GH and dieting. At this
point, whether on a diet or not, T4 has become a much sought after drug
for use with cutting cycles which include Growth hormone. I have even
received a phone call from doctors and Hormone Replacement Clinics, who
wanted more practical information on the use of GH + T4.
So, at this point, I think that most of the people in the bodybuilding
world who are using T4 are using it along with their Growth Hormone for
the most part.
Anecdotally, many bodybuilders feel that T4 is less harsh on muscle
tissue and they're able to maintain more muscle when dieting down by
using T4, than compared with when they've used T3.