IoT healthcare devices often lack resources, and encrypting patient data flows securely is an active area of research. Current cryptographic solutions are typically too computationally and energy intensive to be used on constrained IoT devices. In this work we introduce HealthCrypt: an authenticated encryption scheme for smart IoT deployments in healthcare on highly-constrained microcontrollers. It consists of a novel compact SP-network paired with native authenticated encryption, removing the need to have a separate MAC pass and minimizing total cycle cost. We implement HealthCrypt on an ARM Cortex-M0 running at 48 MHz with the FELICS benchmark suite. Results show HealthCrypt uses only 90 bytes of RAM, only requires 1,620 gate equivalents (under the proposed IoT limit of 2,000 GE) of hardware area, and only uses 5.93 nJ of energy per block encryption. Measurements of its statistical security show it has an almost perfect avalanche of 0.5026, Shannon entropy only 0.0094 bits/byte away from the ideal, and good key sensitivity of 0.4949. Against competitors like PRESENT-80, SPECK-64/128, and SIMON-64/128, HealthCrypt has been rated to have the best overall IoT Healthcare Suitability Score of 79.23.