Several factors can cause brows to thin. Some skin conditions, such as eczema or psoriasis, can cause inflammation. But most cases are related to internal issues such as hormones and thyroid health. A low level of either, and or an off level of either, can lead to hair loss, changes in the condition of the skin, and a whole host of other symptoms. I personally have also experienced this rollercoaster ride!
It is highly recommended to get a metabolic blood test and discuss the results with a trusted practitioner.
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