I'm sorry to hear poor Blossom has been having these issues.
Can I confirm neither her diet nor exercise regime have changed to explain the weight loss?
And that her current issues are: weight gain, darkened (hyperpigmented skin) and itchy skin?
If this is the case, the very top of my list would be a hormonal disorder known as 'hypothyroidism'. This is a condition whereby the body does not produce enough thyroid hormone. This can cause a range of symptoms, including a slower metabolism (which leads to weight loss) and an increase in infections (including skin infections). Other symptoms can include lethargy, patchy fur loss and change in personality (dogs can 'lose their sparkle').
As the skin itself is itchy, it is likely that there is an infection. Sometimes, an overgrowth of yeast can lead to darkened skin.
The first thing we need to do for Blossom is a basic blood test including a thyroid hormone check. Dogs with an under-active thyroid will have low thyroid hormone as well as other changes, such as a mild anaemia and increased cholesterol. If the condition is diagnosed, it can be very well managed with medication.
As I suspect an underlying skin infection, it is likely she will also need a course of antibiotics +/- a medicated wash to treat this. It would also be prudent for us to rule out other causes of the itching, such as external parasites. She should be up to date with a good quality preventative such as Advocate or Nexgard Spectra.
In summary, I suspect a blood test will be needed for us to get our answer. If her thyroid levels come back normal, we would need to go back to the drawing board, but this is the first thing I would want to be looking into.
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