Implanted antennas are a key part of wireless implanted medical devices. However, the lossy nature of the surrounding body tissue significantly reduces their radiation efficiency and gain. Therefore, it is important to maximize the power reaching free space from the implanted antenna. The fundamental modes TE10 and TM10 are of great interest as they have lower near-field losses compared to higher order modes. A Huygens source antenna that combines two first modes, TE10 and TM10, is thus a good candidate for an efficient implanted antenna. However, because the losses for TE and TM modes differ in the implanted scenario, the optimal power distribution between the two modes will vary from that in free space.. Hence, in this contribution we study the theoretical maximum gain and the corresponding optimal power distribution of a Huygens source antenna in a deeply implanted scenario. The encapsulation size and implantation depth, the key parameters affecting the near-field efficiency, are investigated in detail. The Ohmic losses of the electric and magnetic dipole are also discussed for practical design considerations.