I recently adopted a Chihuahua mixed dog from the shelter. Everything is fine, but his lower body (close to the hip) has very thin hair. I suppose she used to bite in that area due to flea infection. Are there any products that would help in hair regrowth in that particular area?
In Chihuahuas, the dilution gene (bluish/dark gray coat) can cause thin coat or even alopecia in certain parts of body - I have seen that on the side of hind legs. However, both parents must have that gene in the first place. If this is the case, you will not be able to completely treat it.
Allergy can be another reason. One of my dogs is allergic to flea bite, and several dog shampoos. The skin gets red, dandruffs appear and there is a specific smell associated with it. Treatment is to first get rid of fleas and then use non-allergic dog shampoo.
Other than that, I'm using salmon oil as a meal supplement. Some instructions also mention direct application on the area (I'm not sure it that works). There are other special products (herbal oils) for skin application, I have used them, but I cannot say if they actually work (no point in naming a brand as it is local product here in Czech Republic).
Hope it at least partially helps.
If your dog hasn't been to the vet for a checkup yet, I would recommend taking her to the vet for an initial health exam and asking about the thin spot. Dogs can have thin hair from allergies, diseases, parasites, or just genetics (as mentioned in the previous answer).
If your vet confirms that the thinning fur is due to genetics or a condition that has healed, there has been some research done with using melatonin to stimulate hair growth.
In a study of 29 neutered (so I assume male) dogs with normal hormone levels prior to supplementation, 62% of the dogs regrew hair with oral melatonin supplementation.
In another study Siberian Husky's were clipped and then treated with topical melatonin (melatonin applied directly to the skin, instead of a pill). This study determined
topical melatonin and brushing had no impact on hair regrowth after clipping normal dogs.
So, if your dog's thin hair is caused by the aftermath of a skin condition that caused her to excessively groom, topical melatonin likely will not help. If your dog's thin hair is caused by something else and you try oral melatonin, it might help.
I would recommend talking to your vet before starting supplementation with melatonin or any other product, especially since a Chihuahua mixed dog is likely to be small compared to other dogs used for these studies. My guess is that it's well tolerated, my vet recommended melatonin for an old injury on one of my cats' ears that hadn't regrown hair and he wasn't concerned about dosage at all. It is a sleep hormone in humans, so it may make your dog sleepy.
Frank LA, Hnilica KA, Oliver JW. Vet Dermatol. 2004 Oct;15(5):278-84. Adrenal steroid hormone concentrations in dogs with hair cycle arrest (Alopecia X) before and during treatment with melatonin and mitotane. Online Abstract.
Diaz SF1, Torres SM, Nogueira SA, Gilbert S, Jessen CR. Vet Dermatol. 2006 Feb;17(1):45-50. The impact of body site, topical melatonin and brushing on hair regrowth after clipping normal Siberian Husky dogs. Online Abstract