1 Temperatures measured under the armpit are not considered as accurate and can be as much as 1 degree F lower than an oral measurement. 2  A temperature below 100.4 F (38 C) is considered a low-grade or mild fever.3  It means that the body is responding to an infection. Temperatures measured under the armpit are not considered as accurate and can be as much as 1 degree F lower than an oral measurement. 2  A temperature below 100.4 F (38 C) is considered a low-grade or mild fever.