Prohormones are physiologically non-active or mildly active precursors of hormones. Body metabolism can convert prohormones into the actual, physiologically active hormone in one or more steps as needed. This is a very complex hormone regulatory system that plays a major role, especially in the activation of steroid hormones.
What is a prohormone?
Physiologically highly effective hormones such as steroid hormones, and to a lesser extent other hormones, must be readily regulated in concentration by the body. Above all, the complex interplay of hormones requires physiological adjusting screws that the body metabolism can operate unconsciously for humans. Prohormones offer one of several possibilities for the metabolism to directly influence the concentration of certain hormones. Prohormones are physiologically inactive or only slightly active. This means that they have no or only a weak hormonal effect and are harmless to the body even in higher concentrations as long as they are not converted into the physiologically active hormone. Prohormones can be regarded as a stockpile of certain hormones that the metabolism can dispose of at any time by converting them into the active form. Prohormones have the greatest importance as precursors for a variety of steroid hormones such as the stress hormones cortisol, corticosterone and the sex hormones testosterone, progesterone, estradiol and many others. The well-known vitamin D3 (cholecalciferol) is also a de facto precursor of the hormone calcitriol.
Function, effects, and roles
The main function and effect of prohormones for the body lie in their potential physiological effectiveness active hormone. Metabolism can influence the effectiveness of certain hormones through several regulatory mechanisms. It can increase the concentration of the specific hormone by converting prohormones and thus achieve a stronger hormonal effect, or it can reduce the efficiency of the receptors in the case of a momentary overconcentration of the hormone, thus dampening the hormonal effect. Prohormones, as inactive resereve hormones, make an important contribution to the regulation of the interaction of hormones, especially steroid hormones. A very well known prohormone is thyroxine (T4), a non-proteinogenic amino acid that can be metabolically converted to triiodothyronine (T3). Triiodothyronine is a hormone that is normally produced in the thyroid gland and is required for a variety of metabolic conversions in the body. In cases of hypothyroidism or complete failure of the thyroid gland, or after surgical removal of the gland, the body is dependent on the supply of the prohormone thyroxine within relatively narrow limits. It fulfills a life-sustaining function. In sports where the focus is on muscle building, it is tempting to switch to prohormones from countries such as the USA, where the sale and use of the substances is permitted, instead of the banned anabolic steroids – which are also banned in Germany. In principle, this is not very advisable, because the body converts the substances into the corresponding steroid hormones and the expected side effects resemble those of steroid hormones.
Formation, occurrence, properties and optimal values
The human metabolism is capable of synthesizing most of the prohormones that the body needs from basic substances ingested with food. In addition, naturally leftover foods are used to ingest already “finished” prohormones, most of which are absorbed in the small intestine. For the synthesis of prohormones of the steroid hormones, cholesterols are mostly used as the basic substance. The determination of optimal values for the concentration of prohormones is not very meaningful, because the required amounts depend very much on muscular activity, general metabolic requirements such as stress load, exercise and other criteria. With a reasonably balanced diet, which also includes the consumption of foods that are as natural as possible, a separate supply of prohormones is not recommended in order to avoid unpleasant side effects, some of which may be harmful to health. Among bodybuilders and athletes with a similar requirement profile, the opinion is widespread that enriching the diet with certain prohormones, which are converted into anabolic hormones, would accelerate the desired muscle build-up.Due to frequently observed side effects, some of which are serious and relevant to health, the sale of these prohormones, as well as the sale of anabolic hormones, is prohibited in the German-speaking countries of Germany, Austria and Switzerland. However, the situation is different for other prohormones such as thyroxine. If there is an underactivity of the thyroid gland (hypothyroidism), the supply of a certain amount of the prohormone thyroxine is vital. In this case, thyroxine is classified as a drug.
Diseases and disorders
Apart from very rare – mostly genetic – metabolic disorders that lead, for example, to a deficiency of specific enzymes or hormones so that certain prohormones cannot be converted or broken down, the greatest risks lie in an overdose of prohormones that promote the formation of steroid hormones. The necessary breakdown of prohormones in the liver can be liver damaging, and it can lead to undesirable androgenization symptoms such as loss of head hair, increase in body hair, acne, and other symptoms due to an overabundance of male sex hormones. Most of the precursors for steroid hormones are listed as prohibited doping substances anyway. For example, too low as well as too high estrogen levels affect a reduced synthesis rate of the control hormone LH in the pituitary gland with the effect of reduced testosterone production. However, when thyroxine supplementation is medically necessary, the amount of the prohormone must be strictly controlled because both overdose and underdose adversely affect a variety of metabolic functions and also adversely affect the psyche.
