The best replacement for testis is testes.

The best alternatives to testis are rated as following:
★★★★★  testes
★★★★  testicle
★★  ovary
★  epididymis
  ovaries
  teste
  cryptorchidism

Your alternative for testis is not listed? Suggest!
Speaking about testis, one probably shouldn't forget about hypospadias.

Other search terms related to testis and their ratings are:
★★★★★ hypospadias
★ fixation of contralateral testis
You have in mind a companion for testis and it is not yet here? Propose!



Google Trends

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testis
testes
testicle