Most cases of telogen effluvium can be diagnosed based on medical history and an examination of the scalp and hair. If the hair loss has been occurring for several months, there may be visible thinning patches, but often the hair loss is not dramatic enough for a doctor to notice. If you have large bald patches, you probably don't have telogen effluvium. If the doctor gently tugs on some hairs on your scalp and four or more hairs come out, you probably have telogen effluvium. Also, the hairs will look like hairs in the telogen phase — they will have a white bulb at the end that was in the scalp, and will not have a gel-like covering around that end of the hair.
You may be asked to gather all hairs that fall out of your head over a 24-hour period, and count them to see if the hair loss is truly excessive. Losing fewer than 100 hairs in a day is considered normal. You also may be asked to gather and count lost hairs every one or two weeks to see when the shedding starts to decline.
In some rare cases, if there is reason to doubt the diagnosis, a biopsy of the scalp may be done. In this procedure, a small piece of the scalp that includes several hair follicles is removed and examined under a microscope. Your doctor also may do blood tests to check for conditions such as thyroid abnormalities that may be contributing to hair loss.