Background: The understanding and description of forbidden decays provides interesting challenges for nuclear theory. These calculations could help to test underlying nuclear models and interpret experimental data. Purpose: Compare a direct measurement of the La138β-decay Q value with the β-decay spectrum end-point energy measured by Quarati et al. using LaBr3 detectors [Appl. Radiat. Isot. 108, 30 (2016)ARISEF0969-804310.1016/j.apradiso.2015.11.080]. Use new precise measurements of the La138β-decay and electron capture (EC) Q values to improve theoretical calculations of the β-decay spectrum and EC probabilities. Method: High-precision Penning trap mass spectrometry was used to measure cyclotron frequency ratios of La138, Ce138, and Ba138 ions from which β-decay and EC Q values for La138 were obtained. Results: The La138β-decay and EC Q values were measured to be Qβ=1052.42(41) keV and QEC=1748.41(34) keV, improving the precision compared to the values obtained in the most recent atomic mass evaluation [Wang, Chin. Phys. C 41, 030003 (2017)1674-113710.1088/1674-1137/41/3/030003] by an order of magnitude. These results are used for improved calculations of the La138β-decay shape factor and EC probabilities. New determinations for the Ce138 2EC Q value and the atomic masses of La138, Ce138, and Ba138 are also reported. Conclusion: The La138β-decay Q value measured by Quarati et al. is in excellent agreement with our new result, which is an order of magnitude more precise. Uncertainties in the shape factor calculations for La138β decay using our new Q value are reduced by an order of magnitude. Uncertainties in the EC probability ratios are also reduced and show improved agreement with experimental data.