The first question is to ask if the cat is losing fur only on the tail or also on other parts of the body. Also it depends on the configuration of the hair loss. Is it small patches, a single larger area or a general thinning? These are all relevant questions in dealing with feline hair loss on the tail.
It would be unusual to just be losing fur on the tail and if that was the case my first thoughts are that it would be over-grooming due to stress or anxiety. It might be useful to watch your cat as much as possible to see if she is grooming her tail more than usual.
The tail is easily accessible and therefore, although over-grooming normally takes place on the belly because that too is easily accessible, it might also occur on the tail. Over-grooming removes some of the fur from the body which leaves bald patches or thinner hair.
There were two medical conditions which affect thyroid production and which can cause hair loss, thinning or balding of the coat. One of them is hyperthyroidism which is in excess of thyroid hormone. About one third of cats with this endocrine condition will have hair which pulls out easily and hair loss.
The other is a deficiency in the thyroid hormone which is hypothyroidism in which causes dry skin and thinning of the hair-coat. The hair becomes dull and brittle but this condition is rare in domestic cats.
It is just possible that it might be ringworm. This is a fungal infection which causes hair loss in small patches. It creates scaly, crusty or red circular patches up to 2 inches across. The patches have central hair loss with a red ring at the periphery. It’s a highly contagious condition and people can easily catch it from their cat.
Allergies cause itching which in turn causes over-grooming. However, parts of the body in addition to or other than the tail would be affected.
There is an allergy, an allergy to flea bites, which might be applicable. It is not uncommon for cats to pull out their hair around the base of the tail if they suffer from this allergy.
In fact, it may be any one of a number of allergies which causes the tail to become itchy but this begs the question whether people are searching on Google for hair loss on their cat’s tail only. A general reaction to an allergen such as an inhaled substance such as pollen should cause itching over other parts of the body resulting in the cat licking those areas as well.
If the hair loss is very localised it may be a simple injury in which the fur has been stripped away from the skin.
The real answer is to see a veterinarian and not to rely on the internet because the topic is too complicated to resolve with certainty from words written without expert help.
This is not necessarily a complete list.