Orach Chayim 61:3 (based on Berakhot 60b) says that the chazan should repeat ה' אלהיכם אמת after the 245 words of the Shema, in order to have 248 words in total, which corresponds to the traditional number of the human body's parts. Rema comments that we add at the beginning אל מלך נאמן instead if we are praying without minyan (even though this practice is not followed in Frankfurt and at Chabad). One should also say the whole Shema before going to bed if he didn't do so after tzeit (Mishnah Berurah to Orach Chayim 239:1).
In almost all books at the bedtime Shema just the first part is present, although אל מלך נאמן is still there, and I've seen old manuscripts, where it is even with big and/or fancy fonts. What is the point for saying these three words if one is going to say only the first paragraph?