Aim: To clarify the immuno-active LH (I-LH) and bioactive LH (b-LH) responses and qualitative changes in the circulating LH to testosterone undecanoate (TU) injection. Methods: Eight men with Klinefelters syndrome were recruited for the study. They received crossover injections of TU at doses of 500 and 1000 mg. Serum I-LH and b-LH levels before and at various time intervals after TU injection were measured and the serum I-LH, b-LH, b-LH/I-LH (B/I) and testosterone/sex hormone-binding globulin (T/SHBG) ratio in LH-responders and LH non-responders were compared. Results: A parallel suppression of serum I-LH and b-LH was consistent with their overall high correlation between each other (r=0.84, P<0.001). Mean serum i-FSH levels were decreased by TU injection at both doses without dose-response effects. LH-responders had lower baseline serum i-LH and b-LH, and higher E2 levels and T/SHBG ratio. There was a quantitative change in serum LH as induced by TU without qualitative change within LH-responders os LH-non-responders. Conclusion: A high loading dose (1000 mg) of TU is important for the initial suppression of LH. With the lower dose (500 mg), repeated injections will be required to attain such LH suppression for the purpose of fertility regulation. The lower baseline serum I-LH level may be an intrinsic characteristic of LH-responders

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