Thermal radiation is usually not considered in the interpretation of laminar burning rates measured by the outwardly propagating spherical flame method. However, it may contribute significantly to measurement uncertainty, especially for model-constraining conditions at lower flame temperatures and higher pressures. The present work derives a conservative analytical estimate of the effects of radiation heat loss, which include radiation-induced burned gas motion, decreasing flame temperature due to conduction to the radiating burned gas, and radiation loss from the flame zone. Detailed numerical simulations covering a range of burning conditions serve to validate this analytical tool. Modeling results from both detailed simulation and the analytical tool show that previous measurements of hydrogen flame speeds at low flame temperature and high pressure are minimally affected by radiation, but also that flames with low flame speeds can be strongly inhibited by radiative loss. Interpretation of both future spherical flame measurements as well as existing measurements exhibiting low flame speeds should include consideration of radiation effects to avoid potentially large uncertainties. To aid in this interpretation, a solver with a graphical interface has been developed for facile implementation of the present analytical result.