Updated October 31, 2014.
Periodically, I receive an email from someone who is wondering if she could be hypothyroid. She has many symptoms of an underactive thyroid, except she's slender, and hasn't gained a pound. A few of the women have even reported that doctors dismissed their concerns about thyroid disease, saying, "Well, if you had an underactive thyroid, you'd be overweight, so I don't see a point in testing you." Patients themselves may feel that they are slender, so they couldn't possibly have an underactive thyroid.
While I frequently cover weight gain and the connection to thyroid disease here at the Thyroid site, keep in mind that weight is NOT a determining factor for diagnosis.

A percentage of patients don't gain any weight when hypothyroid, and, there are even some people who paradoxically lose weight when hypothyroid.

How to tell...

If you're tired, feeling depressed, losing hair, feeling cold, have a low sex drive, or are experiencing any one of the many other symptoms of an underactive thyroid, don't rule out the possibility that you are hypothyroid just because your weight is normal. In fact, make an appointment with your physician, and before you go, review the complete Hypothyroidism Risks/Symptoms Checklist, and take this helpful interactive online quiz now: Could You Be Hypothyroid?