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3,3-diiodo-l-thyronine (T2)

The hormone 3,3-diiodo-L-thyronine (sometimes known as 3,5-diiodo-L-thyronine) (T2) is a derivative of T3.  Interestingly, due to very outdated literature and members of the medical community who are not up to date with the latest studies, T2 has been set aside because even some of the top doctors still believe it is not active.  However, recent research has shown that T2 is very active in terms of the metabolic effects in the body; hence, it has proven medical uses to help lose weight and treat cholesterol problems.  As with most other compounds that were initially created for medical purposes, T2 has found a very wide use in the bodybuilding industry.

The difference between T2, T3, and T4 is simply the amount of iodide atoms each of them has.  For instance, T2 has 2, T3 has 3, and so on.  As the body removes the iodide atoms from the hormone, it will become more active.

3 3-diiodo-l-thyronine (T2)

Fig 1. 3 3-diiodo-l-thyronine (T2)

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Animal studies have shown that T2 increases oxygen consumption by over 25%, so it is more effective than T3 or T4.  What's interesting, this is made possible because T2 acts directly through activating mitochondria.   Another study put T3 vs. T2, to see which one was more effective when it came to resting metabolism and tissue oxidative capacity, and they found that the liver responded most with T2.  Therefore, the conclusion of these studies showed that T2 could be a direct mediator of thyroid hormone regulation on energy metabolism, meaning that it fights fat very well.

Meanwhile, human studies on T2 aren't very numerous yet, but they do exist.  For example, one particular human study showed that T2 increased the rate of respiration on human mononuclear blood cells.

Understanding TSH

TSH stands for thyroid stimulating hormone, and using blood work to test for TSH levels can be a good way to find thyroid gland problems.  This hormone is produced when the hypothalamus (region of the brain) releases a substance called TRH (thyrotropin releasing hormone).  Then, the TRH will trigger the pituitary gland to release TSH, which makes two hormones - T3 and T4, which help control the body's metabolism.

Low TSH means an overactive thyroid gland (hyperthyroidism).  This can lead to a hard time gaining weight (weight loss), sweating, rapid heart beat,  sleep problems, fatigue, and muscle weakness.

The flip side would be hypothyroidism, which is where the thyroid gland doesn't produce enough hormones.  This condition is especially common in older women, and it can lead to constipation, fatigue, weight gain, dry skin, puffy face, slowed heart rate, depression, joint problems, and tender/painful muscles.

T2 effects on TSH

Compared to T3, T2 is less inhibitory on the TSH.  When put against each other, it would take 10 times more T2 to get the same effects as T3 has on TSH.  This means that T2 is probably going to be less catabolic than T3 would be when it comes to muscle waste.  Thus, T2 is safer at not altering your thyroid in the body.  In other words, there isn't a worry about 'shutting down' your endogenous T3 or T4 when you take T2.

Muscle fibers

Slow twitch fibers contain myoglobin (which stores oxygen), they are small and red.  These fibers are used in aerobic exercise because they consume oxygen and fat for energy.  On the other hand, fast twitch fibers are white fibers, since they are filled with glycogen/starch.  These fibers are known for strength, speed, and power.

Now that we have briefly discussed the differences between the different types of muscle fibers, we can talk about T2's effects on them.  There is evidence that T2 turns your slow twitch muscle fibers into fast twitch fibers, which is done by decreasing triglyceride levels.  Furthermore, T2 also stops fat from being deposited into white and red muscle cells, leading to an increase in muscle cell insulin sensitivity.  As a result, you will end up with more amino acids circulating, and more carbs and creatine being absorbed by white muscle cells.  Meanwhile, the red fibers will shrink, and white fibers will grow, causing more red muscle fibers to convert to white.  This will create a permanent increase in the maximum strength and size that your muscles can reach.  Besides, it will also mean harder and dryer muscles that are made swollen by glycogen.


You want to dose T2 at least twice per day, between 50 and 300 micrograms (mcg).  At first, it may seem to be quite a hefty dose, but as mentioned above, T2 should be dosed more than T3.

Where to buy

T2 can be tricky to buy.  There are supplement companies that sell what they claim contains T2 in capsule form, but in reality it is just a bunch of vitamins and minerals.  Additionally, there are other companies that sell a liquid version of T2, which should be held under the tongue and swallowed.


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Steve Smi (Also known as Steve Smith), has over 12 years of experience in the bodybuilding and fitness industry. He's the co-Host of the popular Evolutionary Radio Podcast - over 500,000 listens! He has been a personal trainer and writer for the last 10 years with over 1000 articles written. He's been a moderator on forums for over 10 years. Steve holds the following certifications and records: NASM Personal Trainer NASM Fitness Nutrition Specialist Florida Local Powerlifting champion Top writer for 2019 Fitness rated

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