I guess without Synthroid, I would have no energy at all, but our relationship has been a love/hate one. When I was first prescribed Synthroid many years ago for hypothyroidism, my physician made the mistake of starting me on too high of a dose. I became very jumpy, and developed an irregular heartbeat. It was a very scary feeling, but he didn't seem all that concerned. I finally insisted that I could not tolerate the side effects at that dosage, and finally convinced him to reduce it.
I will always believe that Synthroid is what caused my irregular heartbeat (that I still have.) It was diagnosed by a Holter Monitor that I had to wear for 24 hours. I had thousands of irregular heartbeats in that period of time. They were determined to be "Benign" but frightening nonetheless. Now years later, I still suffer from them occasionally. I can actually feel my heartbeat skip. Certain blood pressure medicines aggravate the heart rhythm, and I have to try something else.
Eventually, I found a doctor who started me on a low dose of Synthroid, and then gradually worked me up to the dose that the first physician started me on. I seem to tolerate it quite well now. If you've ever suffered from an under-active thyroid, then you know that you cannot leave that condition untreated. You simply will not have the energy to live.
So like the title says, for me, Synthroid has been both a blessing and a curse.