I've had Hashimoto's - basically an underactive thyroid with autoimmune issues (Koala77's post is excellent) - for about 15 years (my husband had Grave's until medivac'd from Iceland by the Navy for radio-a treatment to deactivate his thyroid - we've had interesting discussions re: Synthroid for years
In March, '09, 4 months after surgery for a simple broken hip and pelvis, I started losing weight dramatically. Drs 1st wrote it off to inactivity, but I suddenly developed projectile vomiting to the point of not keeping food or water down. Drs started new treatments and also responded to what they thought was Raynaud's (my symptoms were severely painful, pure white hands - an innocent but drastic misdiagnosis).
I then lost 70 lbs in 4 short months, leaving me under 110 lbs at 5'10". My Rheumo arthritis Dr had me rushed to the hospital after 2 blood tests 10 days apart. He was positive that the 1st results were completely misread - they read like something from an autopsy. ER informed my husband that I was 2 hours from death due to organ shutdown caused by severe malnutrition, and to call family rapidly.
Many tests, dollars, and months later, my neurologist pinned it on a "Thyroid Storm". With Hashimoto's and too infrequent blood tests for Synthroid management, other organs "scream" at your thyroid for increased output. Your thyroid "wakes up" and goes into super overdrive like a space shuttle launch. The other organs go into shock from thyroid-output starvation to output overload; a GP will not catch this with longterm Hashimoto patients and blood-testing only during your yearly physical checkup.
Everything happens in what seems like the blink of an eye. The permanent damage of severe malnutrition is irreversible, devastating, and painful beyond belief.
If you do not have them, consider getting a regular endocrinologist and neurologist to help with your Hashimoto management. The best of luck to you, sincerely, in learning to deal with Hashimoto's - just watch out with Hashimoto's, it's sneaky. It can cause a life-and-death situation in an instant and the longer a person has it, like me, the less attention is paid to it, as if it were poison oak from a nice weekend camping trip.
I really hope this helps you and anyone else new to Hashimoto's. I learned, almost too late.