What Causes Severe Cramps In The Feet?

I have severe muscle cramps, usually in calf muscles, which cause my feet to curl, and which cannot be released unless I stand or walk, which is very painful. It mostly happens at night, especially if my feet/legs get cold and/or when I stretch (such as when waking). I often have large twitches in calf and/or thigh muscles in large areas (like a bag of worms). I have had such strong contractions that they caused severe ankle pain in muscle/tendon at lateral calf and ankle that lasted for about a week then finally became chronic pain. I take thyroid medication for hypothyroidism, and recent labs showed TSH in normal range. I used to have similar, less severe symptoms before diagnosed with hypothyroidism (always cold, severe leg cramps when cold at night, etc.). Any thoughts? Thank you, Linda Reinhart