Structure
Structurally trenbolone is a derivative of nandrolone, carrying two additional double carbon bonds at positions 9 and 11 (hence the prefix "tren", short for tri-en). The activity of trenbolone differs from that of its parent hormone considerably however. To begin with, trenbolone cannot aromatize to estrogen. The delta-9 group present on its structure occupies a bond necessary for aromatization of the A-Ring to be possible. Unless this group is removed metabolically, which it does not appear to be, estrogen synthesis is impossible in the body. Although nandrolone is a weak substrate for aromatase, estrogen levels can still rise during use. With trenbolone we actually expect a lowering of serum estrogen levels, as it should suppress endogenous testosterone release (the primary substrate for estradiol in men).
Androgenic Activity
Although derived from nandrolone, trenbolone is comparatively far more androgenic than this steroid. In fact it is several times stronger in this regard than our primary androgen testosterone as well (1). The first contributing factor to this of course is that trenbolone is a strong binder of the androgen receptor. This trait is also characteristic of its parent nandrolone, which is several times more active than testosterone in this regard. Androgen binding is in fact further enhanced by the introduction of double bonds in delta-9,11 (2), which makes trenbolone an even more potent agonist of the androgen receptor than nandrolone. Perhaps more significant though is the fact that unlike nandrolone, the strong receptor binding potency of trenbolone is not diminished in androgen sensitive tissues by 5-alpha reductase. Trenbolone does not seem to undergo 5-alpha reduction in humans to any appreciable degree at all, which is evidenced by the fact that the major urinary metabolites of trenbolone all possess the original tri-en structure with an intact delta-4 group (3). So trenbolone retains its original potency as it enters cells in androgen target tissues with high 5AR concentrations, as this enzyme is not affecting it. These factors work together to allow trenbolone to be a potently androgenic steroid, instead of a primarily anabolic one in nature like nandrolone. .
Progestational Activity
It has been reported in other bodybuilding literature that trenbolone does not exhibit any activity as a progestin in the body. I am not certain where this belief originated, as trenbolone does appear to exhibit the classic progesterone receptor binding ability that is characteristic of nandrolone and its derivatives. One study looking at the bovine uterine progesterone receptor for example found trenbolone to be a very potent binder, startlingly even more so than progesterone itself (4). Another looking at the binding of various compounds to the androgen, estrogen, progestin, mineral corticoid and glucocorticoid receptors found trenbolone to be a more potent binder of the progestin receptor than nandrolone (5), a steroid normally noted for its usual activity in this regard. What does this mean for trenbolone? I don’t think it really means that much. Trenbolone clearly doesn’t cause gyno, water retention or fat buildup, which one might attribute to estrogenic or progestational activity. So whatever slight action it does have as a progestin on paper doesn’t amount to all that much in the real world. The absence of estrogen may be a significant factor, as progesterone is believed to cause gyno by enhancing estrogen’s stimulation of mammary gland growth (6). Perhaps when trenbolone is taken with other aromatizable compounds it could affect a person’s sensitivity level to gyno and water/fat retention. This seems logical, at least in a technical sense, although admittedly I have seen no evidence to support this.
Mass or Cutting Agent
The potently androgenic and non-aromatizing nature of trenbolone makes it an extremely effective hardening and cutting agent. In fact, it is thought of as unmatched in its capacity as a body-sculpting steroid. Many competitive bodybuilders similarly find it indispensable to any good pre-contest cutting stack. For this type of purpose I doubt another steroid would serve you better. Many people do additionally find they make very good muscle gains with trenbolone. It is a potent muscle-builder, although we should probably not consider it an ideal mass-builder when used alone. The absence of estrogen is an important factor, as this trait seems integral in this type of steroid. This probably has to do not only with water retention but also interactions between estrogen and muscle glucose utilization, GH release and androgen receptor proliferation. Today we are finally starting to understand why this hormone is needed for optimal growth. Trenbolone is probably still the most potent muscle-building agent of all the non-estrogenic steroids though, and admittedly is quite unusual in its potency in this regard. But I would still think that if mass were the goal and you were choosing only one steroid, testosterone, Dianabol or Anadrol would be more productive every time in terms of overall size, weight and muscle mass gain.
