My Rhodesian Ridgeback has black streaks on the side of his ridge and large black splotches under his coat around the ridge. Haven't changed his food or introduced anything new. I've researched as much as I can and have diagnosed a thyroid issue. I've never had any luck with the vet and am against giving my pup and myself pharmaceuticals. I'll post a picture. Canaan is 4 years old.
Self-diagnosis is not doing your dog any good. If you aren't happy with your current vet, then I recommend finding another. Skin issues are very difficult to diagnose, even for professionals. Diagnosing on your own and using herbal/alternative methods to treat what you think is the problem can make things worse.
True the RR is known to have a higher than average incidence of problems due to an underactive thyroid. This deficiency can influence a variety of health problems but I have never heard of black markings in the coat being a symptom of this. The most common symptoms are bald patches and general lethargy. Any vet will be able to diagnose thyroid levels with a simple blood test, though they will usually send a second sample to an external lab, to confirm the diagnosis and the dose of replacement hormone needed. Some black hairs are allowed on the head and chest of the RR, but larger areas are not permissible in the show ring. However, purely show points are not important for a pet or sports dog. If you are worried about the significance of these black markings I would contact the breeder. I should add that if your dog has a deficiency in one or more of his thyroid hormones the condition can only be treated by replacing the missing hormone - usually by using a synthetic substitute. Once properly diagnosed, this is an easily managed condition.
I would suggest seeking a second opinion from a different veterinarian. It could be a thyroid issue or it could be something else entirely. Without a proper diagnosis, it's difficult to determine the best course of action. Additionally, there are natural remedies that can be used to support thyroid function, such as supplements and dietary changes. It's important to prioritize your pup's health, so don't hesitate to seek professional help.
Are there any symptoms other than the black markings? Has his behaviour changed in any way? Is he eating/drinking as normal? As Carole said, a thyroid panel (blood test) would rule in/out any issues in that area