What causes hypotension during exercise or during exposure to heat? not dehydration, thyroid, cardiac, pulmonary, or endocrine. bp normal otherwise.

That may be normal: Do you just want to know such phenomenon or do you personally experience symptoms such as dizziness at fast standing-up during exercise? Both exercise and heat exposure normally dilate all blood vessels so to lower blood pressure, but usually autonomic nerve system can automatically regulate the degree of vessel dilation so to maintain circulation to the brain adequately. But, few cannot do so...

Vasodilation: Hypotension is by definition low BP and while BP may be normal you may experience lower BP relative to your normal when your vessels dilate in response to a rise in internal temperature. Cardiopulmonary, endocrine, anemia, dehydration and infection are potential causes, as are medication side effects. Note that you can be volume depleted -- with these symptoms -- and not technically dehydrated.