Iodine deficiency is considered to be a widespread problem in the U.S. nowadays. Is there regular iodine in your diet via seafood and/or sea vegetables? Do you take a supplement that contains iodine/iodide? If not, you may be deficient. Iodine deficiency can affect your energy levels because your thyroid needs it to produce its hormones (though technically, the thyroid wants the iodide form). If you're in doubt, a standardized bladderwrack supplement, with 150 mcg of iodine, should suffice as a safe and effective supplement. However, more might be in order if you're severely deficient, but more is not better if you don't need it.
Doctor's Data offers a urine test whereby you do an iodine load and then collect your urine for 24 hours. The percentage of iodine you excrete helps determine your body's levels. You'd need to find a practitioner -- doctor or someone else who can order tests -- to get this ordered.
If anyone suggests you paint some tincture of iodine on your skin and then check to see how quickly it disappears, don't do it. Most of the iodine evaporates, making it completely unreliable.
Also, if you have fibrocystic breasts or ovarian or uterine cysts, this may also be indicative of iodine deficiency.