What Causes Itching All Over The Body While On Thyroid Medication?

I have suddenly started itching first in my hands and then my feet. It then has spread to other parts of my body. It's more than dry skin it feels like pin pricks. I had blood work done nothing unusual showed up. It gets worse when I have a hot flash. I take thyroid medication. What is causing this itching? I'm taking antihistamines to help with the itching
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