Breast itchiness should first have several factors to take into consideration.
-is it localized to one area?
-is it only one breast and not the other or both?
-is it associated with something you can feel underlying the area- like a bump or lump?
-is it itching by itself or is there are rash you can see?
If itchiness is on both breasts all over, no rashes or lesions, and not localized to one area, I often see contact dermatitis as a cause. Reactions to the material of bras as well as to detergents that may not be completely rinsed out in the wash can occur.
We come across yeast folliculitis from sweating excessively as well.
Heat rash from excess sweat and heat from menopausal flushes can also lead to this common symptom.
If the breasts are itchy overall, hormones can play a role. This is possibly due to swelling of the breasts that can lead to itchiness and irritation. Common times this may be seen is during puberty and during pregnancy.
If itchiness or discomfort is localized to under both breasts, perhaps more concentrated on the side you sleep on: Yeast infections in general can also cause itching and discomfort but tend to be localized to the area under the breasts.
Eczema, dry skin and psoriasis can also impact the breasts as well and may even be localized to the areola and nipples.
I have come across some less common causes as well including a genetic condition that can occur later in life called Hailey Hailey disease.
For all of the above mentioned, initiating intervention of using topical steroids, moisturizers, trying different materials for bras, cooling the skin can often help.
Over the past 20 years I have seen my fair share of breast cancer presenting on the skin. I would strongly advise that if itchiness lasts longer than 2 weeks and the interventions listed above do not work or if there is an underlying lesion, lump/bump, rash, discoloration, tenderness, localized to one spot - see your doctor. I have a lowered threshold to biopsy the breast given the fact that I have seen inflammatory breast carcinoma present as a simple red patch, Paget disease that looks like eczema. The key with each of these is a lack of response to treatment over 2 weeks or lasting response over a month.