This paper investigates physical layer security (PLS) of uplink non-orthogonal multiple access (NOMA) in Internet of Things (IoT) networks, where two devices transmit secret information to base station or access point simultaneously under the intercepting of an eavesdropper (Eve). The new closed-form expressions of joint connection outage probability (JCOP), joint secrecy outage probability (JSOP) and sum secrecy throughput (SST) are derived over Nakagami-m fading channels to indicate the impact of parameters, i.e., transmit power and codeword rates, on secrecy performance. Simulation results are obtained to verify our analysis and allow an easy design of transmit power and wiretap code rates which maximizes the SST.