Have had a purple mark on lower right leg for several weeks...>in size when pressed/< when released. not a bruise...what could it be? concerning?

Might be a clot,,,: This might be a clot in your leg vein from a swollen vein, like thrombophlebitis or from venous stasis. This is seen especially when someone stands for long periods without a sitting break. Also, your age and history of hypothyroidism might predispose to this. It would be smart to see your doctor soon to get this examined and treated so it doesn't grow or result in a pulmonary embolism.