Due to an issue in how the app measures blood glucose levels, Apple will be temporarily removing the blood glucose monitoring feature while it works on a fix, according to CNET (via MacRumors).
Health is Apple's dashboard app for displaying and tracking various health metrics such as calories burned, heart rate, cholesterol, and others including blood glucose levels. It was introduced in iOS 8.
In regards to the issue that forced Apple to remove the blood monitoring feature, a bug is apparently preventing the app from recognizing the two different unit measurements that users plug into the app. The app recognizes mg/dL (milligrams per deciliter), but it doesn't recognize mmol/L (millimoles per liter).
To avoid confusing users, Apple will simply eliminate the ability to input new blood glucose metrics until an update can remedy the problem.
Apple addressed the temporary removal on its support page, promising to maintain the glucose metrics within the app while working on a fix, which will allow third-party apps to continue "talking" with the Health app's glucose levels, even though inputting new information will be limited.
If you have previously entered values manually in the Health app, you'll no longer see this data in the Health app after the update. However, your data won't be deleted, and other apps with permission to read health data will still have access to blood glucose values that you previously entered.
Third-party apps will continue to be able to support both units of measurement and can continue to use HealthKit APIs to store blood glucose data.
This is the second hiccup that Apple has faced with its Health app.
Upon the release of iOS 8, Apple had to remove the HealthKit functionality from the Health app, which prevented other third-party apps from sharing health metrics between each other.
A fix was later introduced that restored the HealthKit functionality in iOS 8.0.2.