Can a forehead reduction lead to hair loss or hair thinning?

You should first make an appointment with a doctor for an exam to make sure a forehead reduction is possible in your particular case.The surgery can cause hair loss and thinning.  It is a risk of surgery but in general the hairs grow back after several months.  This and other risks are something your doctor can explain to you in detail.

Hair loss secondary to hairline advancement? If you are a male, this is not the procedure for you! If you are a female then there has not been a problem of thinning over my 40 years of developing this technique and thousands of surgeries.

A forehead reduction can lead to hair loss and/or hair thinning depending
the amount of scar tissue. Please go back to your plastic surgeon and review
all your photographs. You may need Fraxel laser to decrease the scar tissue.

There are two types of procedures for lowering a hairline. One, is a surgical excision of the upper forehead followed by the downward advancement of the hairline up to an inch from where it was prior to the surgery. To accomplish this, the entire scalp from the frontal hairline to the very back of the crown, is lifted up and pulled forward. When this is done by an experienced surgeon, the hairline can often be advanced about 2/3rd – 1 inch if there is enough looseness when this lifting process is done. This procedure is often complete in one surgical session; however, it may not address receded corners of the hairline if the corner recession is significant. A great advantage of this surgical approach is the immediate impact of the procedure on hairline location with healing usually complete in a week or so. Sometimes this procedure produces a scar in front of the hairline, and if this is this is the case, a hair transplant in front of the hairline can address the scar. If there is corner rounding or if you are a male, a hair transplant lowering surgery is better because of the risk of recession in men nwith genetic balding. Recession will reveal the scar from a hair line advancement surgery. The only disadvantage to this is that it takes 6-8 months to fully grow out. For men, a hair transplant is a better procedure because of the risk of hair loss is high and hairline recession will show the scar from a hairline advancement surgery..

If you are experiencing progressive hair loss or have had surgery such as face lift, forehead lifts and hair transplants, you should consider forehead reduction with caution. 

Progressive hair loss after hairline lowering has not been a problem in my experience after we developed the Irregular Trichophytic Hairline Incision technique many years ago.