I was reading recently that being hypothyroid, your cells can have problems with converting sunshine to Vitamin D, hence the reason hypothyroid people who live in sunny countries still test low for Vit D. Suzy Cohen mentioned it in her book, Thyroid Healthy.
Also, you need UVB rays to make Vit D, and controversial to the sunsmart message, these are strongest when your shadow is shorter than you (so midday). Morning and late afternoon is when its mostly UVA rays. UVB rays are very weak, unlike its aggressive counterpart UVA, so it can't pass through glass (windows etc), not even a tissue, so sunblock blocks it out, and there definitely won't be any exposure sitting in the shade.
Of course, sunBURN is still bad, and too much sun on your face isn't advised cos the skin is thinner, therefore increasing wrinkles, drying out the skin, etc. Our bodies can make 15-25,000IU within 15-20 mins of being in the sun, depending on the amount of melanin in your skin, so I wouldn't stress it over 1000IU or 2000IU. Choose whichever sets your mind at rest. I personally take 5000IU a day as an all year round maintenance dose, because I know I don't actually get as much UVB sun as I'd like to think I do. Being hypothyroid, I can't tolerate too much heat (although I love being warm for a change!!), and I don't like getting too much of a tan as my skin is dry enough as it is... Hopefully now that I'm finally on meds, that might all change.