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Objective: In addition to its skeletal effects, vitamin D may also be impo rtant for health in general. It is uncertain what level of serum 25-hydroxyvitamin D (25(OH)D), marker of vitamin D status, is sufficient for these effects. With decreas ing serum 25(OH)D levels there is an increase in serum PTH. The point at which this occu rs has been considered as a threshold for vitamin D sufficiency. The thresholds found have varied widely and have mainly been based on observational studies. However, to truly e stablish a threshold for vitamin D effects, this has to be based on randomized controlled trials (RCTs). Methods: The study included 2803 subjects from a general health survey, the Tromsø study, and pooled individual person data from five vitamin D int ervention studies (n = 1544). Serum parathyroid hormone (PTH) and change in PTH after vitamin D supplementation were related to serum 25(OH)D levels in steps o f 25 nmol/L (<24, 25–49, 50–74, 75–99, and >99 nmol/L). Results: In the Tromsø study, in the females there was a gradual decrease in serum PTH with increasing serum 25(OH)D with no apparent plateau, whereas in the males the decrease in PTH in subjects with serum 25(OH)D >74 nmol/l was m arginal. In pooled RCTs, there was a significant reduction in serum PTH by vitamin D supplementation regardless of baseline serum 25(OH)D level. Conclusions: The use of the serum PTH–25(OH)D relation from observational s tudies to determine a threshold for vitamin D sufficiency is highly questio nable.