Plasma testosterone and androstenedione levels in men were measured after oral administration of free testosterone and testosterone undecanoate. Both androgens were determined by simultaneous, specific radioimmunoassays after separation and isolation by thin layer chromatography. While free unesterified testosterone had no effect on plasma androgen levels, a striking increase of both testosterone and androstenedione levels was noted after administration of testosterone undecanoate, which is otherwise only achieved by parenteral testosterone application. This effect of testosterone undecanoate is probably due to absorption via the lymph rather than via the portal vessels so that peripheral circulation is reached before metabolism in the liver. Testosterone undecanoate promises to be an effective medication for oral androgen replacement.

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Effects on plasma T, DHT, A, FSH, LH and PRL concentrations and the pituitary responsiveness to the LHRH/TRH stimulation, as well as on libido and sexual, mental and physical activities were studied in ten hypogonadal male patients undergoing therapy with oral testosterone undecanoate (TU) for 9 weeks. The daily dose of TU needed for satisfactory clinical effect was 120 mg (6 cases) and 240 mg (4 cases). There was a significant rise in circulation androgen concentrations in all patients. Mean plasma T, DHT and A increased at 9 weeks from 1.20, 0.12 and 0.36 ng/ml to 4.34 (P less than 0.0004), 0.39 (P less than 0.0004) and 1.24 ng/ml (P less than 0.005), respectively. In hypergonadotrophic patients (N = 6) plasma FSH and LH fell progressively (P less than 0.05), while in hypogonadotrophic patients (N = 4) a marked rise in plasma FSH (P less than 0.05) was found, while LH tended to rise as well. Base-line plasma PRL remained unchanged. In three out of four patients with poor PRL response to TRH, normal responses were established after TU therapy. Increase in libido and sexual activity was reported by nine patients. An increase in mental and physical activity was found in seven and two patients, respectively. Tolerance was excellent. It was concluded that oral TU is an effective form of substitution therapy in male hypogonadal patients.

Thirty‐four hypogonadal male patients were given testosterone undecanoate (TU) orally for 8 months in a dosage of from 40 to 160 mg/day. Before the start of, and during, the study the patients were interviewed about their libido and their sexual, physical and mental activities, and plasma levels of testosterone (T), 5α‐dihydrotestosterone (DHT), oestradiol (Oe2), FSH, and LH were measured by radioimmunoassays simultaneously. Prostate gland was palpated regularly and a routine laboratory screening programme was also performed. All patients reported a marked increase in libido and sexual, physical and/or mental activities and preferred TU over injectable preparations. Circulating T, DHT and Oe2 rose significantly in all 34 patients and the levels remained stable throughout the whole study. The mean plasma FSH and LH levels in the hypergonadotrophic patients were significantly lowered, but remained above the upper normal limit. In the hypogonadotrophic patients, mean plasma FSH levels increased, but not significantly, while mean LH levels remained unchanged. Neither changes in the size and consistency of the prostate gland, nor abnormal values during the routine laboratory screening programme were observed.

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