Climatic conditions may impact health, performance, and well-being of dairy cattle, especially during early life. The objective was to assess the effects of temperature-humidity index (THI) at birth on serum total proteins (STP) concentrations and on the time to the first diarrhea event during the initial 80 d of life of organic certified Holstein heifers. A retrospective analysis was performed on calves born between June 2017 and June 2018, with a total of 420 heifers included. STP concentrations (mg/mL) and diarrhea events from birth to weaning were obtained from farm records. Temperature and humidity were continuously measured during the study period using automated loggers (Onset Computer Corporation, Bourne, MA). STP concentrations were categorized in 3 levels (1 [<5.5; n = 11]; 2 [5.5–7.5; n = 323]; and 3 [>7.5; n = 86]). In addition, calves were grouped considering the maximum THI at their day of birth into THI1 (<55; n = 122); THI2 (56–72; n = 168); and THI3 (>72; n = 130). Data were examined using survival analysis for the time to the first diarrhea up to 80 d of life. Wilcoxon test was used to determine significant differences and hazard ratios (HR) by STP and THI categories were calculated. In addition, ANOVA was performed to detect differences in STP for calves born at different THI categories. Overall, THI at birth had a significant effect on the time to the first diarrhea episode (P < 0.0001) with a median survival time for calves in THI3 of 12 d, contrasting with THI1 and THI2 (37 d and 33 d, respectively). The HR for diarrhea presentation for calves born in THI3 was 1.76 times the HR of calves born with THI1 (P = 0.0008). Contrary, STP was not affected by THI in the study animals (P = 0.44). The presented results are part of a project to determine STP measurements and THI linked to early disease and culling of dairy calves for northern Colorado. From this initial analysis, we conclude that THI > 72 at birth had an effect on the health of calves, increasing the hazard of diarrhea presentation.