This is one of the most common questions I get in my regular clinical practice, and it is one of the most misunderstood and misrepresented issues in all of medicine.
The most common form of thyroid disease is hypothyroidism, meaning that the thyroid is under-active. In hypothyroidism, the thyroid gland makes less thyroid hormone than is normal, and symptoms or problems may arise from this. Some people have what is referred to as “subclinical” hypothyroidism, meaning that laboratory studies show they are hypothyroid, but they have no real symptoms. When symptoms do develop in hypothyroidism, they include fatigue and malaise, dry skin and hair, thinning of the hair, constipation and feeling cold even when other people feel comfortable. Many people find that they are not as intellectually sharp as they think they should be, and depression may occur. These symptoms improve tremendously or resolve completely when one is given thyroid hormone replacement. The most common form of thyroid hormone replacement is levothyroxine, which comes as many brands, including most commonly Synthroid and Levoxyl.
One can also have hyperthyroidism, meaning the thyroid gland is making too much thyroid hormone. The symptoms of this condition include a fast heart beat or palpitations, tremulousness, oily skin or hair, frequent bowel movements and feeling hot when other people are comfortable. When severe, hyperthyroidism often leads to weight loss, but the other symptoms are usually bad enough that the person is happy for treatment, even if the weight comes back on.
Because most symptoms of hypothyroidism are generally the opposite of the symptoms in hyperthyroidism, people often conclude that hypothyroidism leads to weight gain. In fact, this is not usually the case. Most people who develop hypothyroidism do not have significant weight gain and treating hypothyroidism usually does not result in much weight loss. If someone has suddenly gained weight without any change in what they are eating or how much they are exercising, and they are newly diagnosed with hypothyroidism, treating the under-active thyroid may help the person return to his or her former weight. Similarly, if someone has dramatically improved his or her diet or exercise habits and has not lost weight, it is reasonable to do blood tests to check thyroid function, but in most cases this does not lead to a diagnosis of a thyroid problem.