Until the 2000s Testosterone (T) Replacement Therapy (TRT) wasn't very satisfactory for male hypogonadic patients because the available T formulations weren't able to reproduce the physiological pattern of T secretion in man. In fact, oral formulations (oral undecanoate T) showed very short half-life (<24 hours), requiring the administration of several daily doses, whereas the old injection products (T esters) were characterized by very long half-life (>7 days) because of their adipose tissue storage, requiring to be administered every 2-3 weeks but determining remarkable and quick fluctuations (in 2-3 weeks) of the testosteronemia with variations in a few days from over-physiological levels (> 2000 ng/dl) to very low levels (< 200 ng/dl). Nowadays, several compounds can attain the standards of suitability and effectiveness of TRT in hypogonadal men. Both transcutaneous (gel) T and long-acting injectable formulations are the most modern preparations that can satisfy the criteria of an ideal chronic replacement therapy. In fact, they keep the serum T levels in the physiological range imitating its circadian rhythm, leading to the development and/or the preservation of male sexual characteristics and, finally, positively influencing bone mass, skeletal muscle and adipose tissue distribution. In particular, the availability and use of long-acting injectable undecanoate T can really improve the patients' compliance as requested for a life-long treatment. However, definitive and conclusive evidence regarding the main end-points, such as the diminished recurrence of falls in elderly men, the decrease in fractures in osteoporotic subjects, the reduction in disabling conditions and the extension of life, have not been reached so far. Therefore, the aim of this review is to sum up the most important evidence that has been collected regarding TRT, highlighting in particular those concerning both transcutaneous and long-acting injectable T compounds.

Serum inhibin B (IB) and testosterone (T) levels, secreted by Sertoli cells (SC) and Leydig cells (LC), respectively, are parameters of the functional state of these cells. Whereas LC activity and, consequently, T secretion are regulated by serum LH, factors regulating IB secretion by SC are still partially unknown. There is evidence that under certain conditions such as puberty, aging or some spermatogenesis defects, LH levels or Gn-independent factors might contribute to regulating SC activity and IB secretion. Among these factors, GH and IGF-I as well as PRL might have a role. Therefore, in order to explore the possible effects of either LH alone and FSH alone or a combination of both Gn, respectively, on SC function, IB plasma levels and spermatogenesis, we studied their effects in 6 patients with hypogonadotropic hypogonadism (HH), whereas the effects of GH on these parameters were studied in 6 men with panhypopituitarism (PH). Finally, the possible effects of PRL on SC function and spermatogenesis were studied in 6 patients with hyperprolactinemia (HPRL); 24 normal, fertile adults served as control group. In men with HH, neither human chorionic Gn (hCG) nor FSH, respectively, were able to increase serum IB after 3 months of therapy, whereas combined Gn therapy for 24 months increased IB plasma levels and stimulated spermatogenesis in 4 out of 6 hypogonadal men. In panhypopituitaric men, GH added to the classical Gn therapy did not have an additional effect on serum IB levels or spermatogenesis. Surprisingly, in our hyperprolactemic men, IB plasma levels were increased and positively correlated (p<0.01) with serum PRL levels, whereas normalization of the latter by cabergoline treatment caused a decrease of IB levels and a moderate increase in T, LH and FSH. In conclusion, the lack of SC response to FSH therapy alone, as opposed to the response to combined Gn therapy, might indicate that normalization of serum T by hCG is required to obtain IB secretion by SC. Addition of GH did not affect SC function, serum IB levels or spermatogenesis. Finally, our data suggest that PRL plasma levels might have a direct role on IB secretion, suggesting that the hypogonadism found in patients with HPRL might be a consequence of both central (inhibition of Gn secretion) and peripheral (stimulation of IB secretion) origin.