This answer to this question is difficult to be specific on Female Pattern Thinning as it depends on the hair type and the strength of inheritance.
The correct medical term for when the anterior region (upper) of the hair becomes progressively finer over time is Androgen Dependant Alopecia.
The textbooks suggest that an individual becomes aware usually when there has been a 30% reduction in volume in the region, meaning hairs per square inch. This is sometimes also pointed out by a friend or hairdresser.
Interestingly this does not mean for women there is an actual loss in hair number, merely that the hairs have been subjected to the effects of the circulating androgens inherited from parent or grandparent, which reduces the quality of the hair. This causes the scalp to become more transparent, as the hairs here now do not provide the coverage they once did or indeed that the sides and rear of the scalp still have.
If I saw a patient in consultation at Sussex Trichology and they were for example 65 years old and had been hair thinning for 20 years, they may have 50% of the volume they once had, untreated this is likely to get a little worse. After the menopause the scalp becomes less sensitive to DHT and creating a degree of improvement in density becomes much easier. It is vital that the correct HRT medication if taken is the style that helps rather than exacerbates. I can guide you and your Doctor gently through this.
Obviously, we all have fine, medium, coarse or Afro hair in type and the concern is all about the patients’ perception of the problem. On occasion, I see patients who perceive the problem to be much worse than it is. Other conditions such as Telogen Effluvium can reduce the volume but usually diffusely.