It is very important for everyone to understand the difference between the types of compounds used for PCT. They are categorized as follows:

SERMs - Selective Estrogen Receptor Modulators
These are compounds such as Nolvadex (Tamoxifen), Clomid (Clomiphene), Fareston (Toremifene), Evista (Raloxifene) that all act to block Estrogen at Estrogen's target receptor sites in certain tissues (namely breast tissue). This process is known as Estrogen antagonism, which is descriptive of the Estrogen blocking action of these compounds, whereby they attach to Estrogen's target receptors thereby forcing Estrogen out of the receptor sites, or unable to bind to these receptor sites. This is what is known as Estrogen antagonism. Within other tissues, such as the liver, they act as Estrogen agonists, having Estrogenic effects themselves in these tissues as evidenced by the positive effects on cholesterol values with some of these compounds. Specifically for the purpose of PCT, many of these SERMs, to varying degrees, will act as Estrogen antagonists at the pituitary gland, causing an stimulation of gonadotropin (LH and FSH release), and subsequently Testosterone as a result. Although SERMs might block the effects of Estrogen in certain tissues, they do not reduce total circulating levels of Estrogen in the body.

See these profiles for more information,

Nolvadex

Clomid

AIs - Aromatase Inhibitors

Aromatase inhibitors are compounds such as Aromasin (Exemestane), Arimidex (Anastrozole), Femara (Letrozole), and Proviron (Mesterolone) as well as many other compounds. Each serve to eliminate total circulating Estrogen levels at the root cause: aromatization. Aromatization is a metabolic process whereby Testosterone (or any androgen capable of undergoing this process) is converted into Estrogen via interaction with the aromatase enzyme. The aromatase enzyme is responsible for the aromatization process, and it becomes inhibited (or disabled) by aromatase inhibitors. Some of these inhibitors, such as Arimidex and Letrozole will only temporarily inhibit the enzyme. Others will do this permanently, such as Aromasin, which are known as suicidal inhibitors. Many of these aromatase inhibitors will vary in strength from very mild, such as Proviron, to very strong, such as Letrozole. Specifically for PCT, AIs serve to alter the negative feedback loop of the HPTA by way of reducing circulating levels of Estrogen. If excess Estrogen is detected in the bloodstream, the hypothalamus will respond by ultimately reducing or eliminating Testosterone secretion, but if Estrogen levels are abnormally below normal physiological levels, it causes an antagonizing effect whereby the HPTA will generate higher levels of Testosterone ultimately. Many studies have demonstrated upwards of 60% or more in increases of endogenous Testosterone production by way of aromatase inhibitor administration.

See these profiles for more information,

Aromasin

Armidex

Letrozole

Gonadotropins

Gonadotropins include HCG (Human Chorionic Gonadotropin) which can essentially be considered exogenous synthetic gonadotropins. These mimic the normal endogenous gonadotropins secreted by the pituitary gland, such as LH (Luteinizing Hormone) and FSH (Follicle Stimulating Hormone). HCG is a peptide (or protein) hormone manufactured by pregnant women and extracted from the urine of these pregnant women, and this hormone contains a subunit in its quaternary protein structure that is 100% identical to LH, allowing HCG to then act on the same receptors at the Leydig cells in the testes to stimulate Testosterone production. Note that the use of HCG, because it acts as LH, will suppress endogenous LH secretion by way of the negative feedback loop which all endocrine glands work on.

Steroidal gonadotropin stimulating agents
These are other compounds that are commonly steroidal in nature (meaning they posses the typical four-ring carbon steroid structure), such as Teslac (Testolactone). They are for the most part few in number and rare in nature but those that exist do exhibit stimulating effects on the pituitary gland, causing LH and FSH release. These compounds might also include aromatase inhibiting activity.

It is very important to understand the compounds and categories of these compounds involved in restoring endogenous Testosterone production in order to properly recover following the termination of an anabolic steroid cycle. For a more complete and all-inclusive guide to proper PCT and hormonal recovery following a cycle, please see the PCT article here: http://www.steroidal.com/post-cycle-therapy/