I have had this problem, when first getting out of bed, for many years. Some advise it is due to under medication of Hypothyroidism, others say it is an imbalance of medication. My symptoms have improved as I perfected my perfect doses of synthetic medication and NDT. I have just experienced how vital the BALANCE is. For 4 days recently I mistakenly under dosed synthetic T4 by 25mcg. Within that time I went from having no symptoms to hobbling like a really old woman. (For the record, my normal daily dose is
T4 - 67.5mcg and T3 -16.5mcg.)
I hope this is of help.