BACKGROUND: Occupational factors have previously been mentioned as contributing to decreased kidney function and the development of chronic kidney disease of unknown cause. Sea salt workers are one of the occupations facing high outdoor temperatures and a highly, intensive workload. OBJECTIVES: The purpose of the study was to examine whether the kidney function of sea salt workers at the beginning of the harvest season differs from kidney function at the end of the harvest season and to identify factors that can predict the change of kidney function. MATERIAL AND METHODS: Data were collected from salt workers (n=50) who were between 18–60 years of age without hypertension, diabetes, and kidney disease in Samut Sakhon province, Thailand. Urine specific gravity (USG) was used for hydration status and the estimated glomerular filter rate (eGFR) was used to measure kidney function. The mixed model was used to find differences over the harvest season and prediction of factors. RESULTS: On average, the eGFR was estimated to decrease by 15.2 ml/min/1.73 m2 over the harvest season. The decline in eGFR of sea salt workers with moderate and heavy workloads were significantly faster than their light workload counterparts after controlling for other covariates. Similarly, dehydration (USG ≥ 1.030) significantly accelerated the rate of kidney function loss. CONCLUSION: Our study confirmed exposure to heat over the harvest season leads to decreased eGFR in sea salt workers. The rate of change of eGFR could be predicted by workload and hydration status. Workers with dehydration who performed medium to heavy workloads in farms showed faster kidney function decline than those who performed light workload.