There are a number of different esters of testosterone, including the commonly prescribed injectables of testosterone enanthate, and testosterone cypionate , as well other esters, such as acetate, propionate , phenylpropionate, isocaproate, caproate, decanoate, and undecanoate.
Each of these different esters is a molecular chain composed of carbon, hydrogen and oxygen atoms.
The main difference between the different esters is how many carbon and hydrogen atoms make up the chain.
For example, the propionate ester is composed of 3 carbons, 6 hydrogens, and 2 oxygens, whereas the cypionate ester is composed of 8 carbons, 14 hydrogens, and 2 oxygens.
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Testosterone Types and Delivery
Overview
In testosterone therapy , testosterone (often called "T" for short) can be administered into the body in a number of ways. The most common method is intramuscular (IM) injection with a syringe. Other delivery methods include transdermal application through gel, cream, or patch applied to the skin; orally by swallowing tablets (this method is uncommon as it has been shown to have negative effects on the liver); sublingually/buccally by dissolving a tablet under the tongue or against the gums; or by a pellet inserted under the skin. The T-delivery method used will depend on the type of medication available in the country of treatment, the health risks/benefits for the patient of the delivery method in question, personal preference, and cost.
Testosterone is not stored by the body for future use, so in order to maintain healthy levels, it must be administered in timed intervals, and in appropriate dosages. Injectable and subcutaneous T pellets remain active in the body the longest. Injectable T is typically administered between once a week to once every three weeks, and subcutaneous T pellets are replaced every 3-4 months. Transdermal T (patch, gel, or cream) is typically applied to the skin in smaller daily doses; oral and sublingual/buccal T are also typically taken daily.
"Normal" testosterone Levels
An individual's testosterone levels are usually confirmed through a blood test called a "serum total testosterone test." Testosterone exists in your bloodstream in two forms-- "bound" testosterone and "free" testosterone. The majority of bound testosterone in the body is chemically bound to a protein called "sex hormone binding globulin" (SHBG). The remaining bound testosterone in the system is mostly bound to albumin, another protein. Free testosterone is considered the "active" form of testosterone, as it is not chemically attached to any proteins; thus, it is readily available to bind to androgen receptor sites on cells.
A serum total testosterone test measures the total of these two forms of T. What are considered normal test levels of combined bound and free testosterone in male bodies can range anywhere from 300-1100 ng/dl (nanograms per deciliter). Levels will vary with age and individual factors.
It is useful to also measure the level of free testosterone in the system, as this may be more indicative of how hormone therapy is progressing. Levels of free testosterone can range between 0.3%-5% of the total testosterone count, with about 2% considered an optimal level. Ask your doctor to check for both total and free levels of testosterone in your system.
A note of caution about greatly increasing your T dosage
During the first months of T therapy, many men feel impatient waiting for changes to happen. Some may consider doubling or tripling their dose, thinking that the more they put in, the faster the changes will come. However, dramatically increasing your dose might have the effect of slowing your changes. This is because excess testosterone in your body can be converted into estrogen by an enzyme called "aromatase." This conversion is part of the body's natural feedback system-- if there is an abundance of testosterone in the body, it is converted ("aromatized") to estrogen in order to maintain a "normal" hormonal balance. Therefore, taking very large doses of testosterone might not be a great idea. Be patient; if you are not seeing results in a reasonable period of time, and/or your T levels are low, discuss modifying your dosage with your doctor.
Testosterone esters: what they are and how they work
Much of the testosterone that is prescribed for the purposes of hormone therapy is in the form of testosterone "esters." An ester is simply a name for a chemical compound that is formed from reaction between a carboxylic acid and an alcohol. A simple chemical diagram of this reaction is shown below in Figure A. Figure B shows the chemical structure of free testosterone (chemical formula C19H28O2) as well as two different esters of testosterone (testosterone cypionate and testosterone enanthate ).
There are a number of different esters of testosterone, including the commonly prescribed injectables of testosterone enanthate and testosterone cypionate , as well other esters such as acetate, propionate , phenylpropionate, isocaproate, caproate, decanoate, and undecanoate. Each of these different esters is a molecular chain composed of carbon, hydrogen and oxygen atoms. The main difference between the different esters is how many carbon and hydrogen atoms make up the chain. For example, the propionate ester is composed of 3 carbons, 6 hydrogens, and 2 oxygens, whereas the cypionate ester is composed of 8 carbons, 14 hydrogens, and 2 oxygens.
Esterification of testosterone is done in order to improve the solubility of testosterone in oil, which in turn slows the release of the testosterone from the site at which it enters the body.
Testosterone, in its free, non-esterified form, has poor solubility in either oil or water-- though it can be suspended in water. Non-esterified testosterone is available in an aqueous injectable form with the drug name "Aquaviron." However, this form of testosterone stays active in the body for only a very short period of time (only a matter of hours, which is explained further below). Because of this, it must be injected on a daily basis in order to maintain a continuous level of testosterone in the blood. Therefore it is rarely used for testosterone replacement therapy as an injectable.
Once you have added an ester group to testosterone, it becomes even less soluble in water and more soluble in oil. Additionally, as a general rule, the more carbon atoms there are in an ester, the more soluble the ester is in oil. For example, testosterone propionate (with 3 carbon atoms in the ester group) is less soluble in oil than testosterone cypionate (with 8 carbon atoms in the ester group). Remember, this is general, simplified rule for our purposes herein; the solubility of a molecule depends on structural factors that are beyond the scope of this section.
So, generally, the more carbons the ester group has, the more soluble in oil it becomes, and the less soluble in water. The term for this ratio between oil and water solubility is called the "partition coefficient"-- the higher the solubility in oil, the higher the partition coefficient.
The partition coefficient of the ester in question is important because is effects how long the drug itself stays in the system. If the testosterone transfers too quickly from the oil to the blood, the result is a sudden spike in testosterone which then rapidly drops once the dose has been used up. In the example of free testosterone injected into the muscle from a water suspension (as in Aquiviron, mentioned above), the testosterone is essentially immediately available to the bloodstream due to its low partition coefficient, and thus there is an immediate spike of testosterone which is used up quickly in the body.
Testosterone cypionate, on the other hand, has a high partition coefficient. When injected into the muscle, the drug remains in its esterified form in a deposit in the muscle tissue. From there, it will slowly enter the circulation as it is picked up in small quantities by the blood. Once the esterified testosterone is brought into the blood stream, "esterase enzymes" cleave off the ester chain in a process known as "hydrolization," thus leaving the testosterone in its free form to perform its various actions and effects.
When people speak of whether a particular testosterone ester is "fast acting" or "slow acting," they are usually referring to the partition coefficient/solubility in oil. As described above, esters with more carbon atoms will generally be more soluble in oil-- they are often referred to as "slow-acting" esters (they stay active in the system longer). Esters that are less soluble in oil are often referred to as "fast-acting" forms of testosterone, referring to the fact that they are more quickly available and used up in the blood stream.
For men who are using injectable testosterone, slow-acting esters tend to be preferred, as fewer injections are needed over time to keep the blood levels of T reasonably constant. Testosterone enanthate (7 carbons) and testosterone cypionate (8 carbons) both take about 8-10 days to be fully released in the system, and so they are typically injected once every 7-14 days. Testosterone propionate (3 carbons) takes about 3-4 days to be fully released in the system, and must be injected in smaller doses at least weekly if not twice weekly. For this reason it is not often prescribed for men in transition.
