Weight gain is a symptom of hypothyroidism. The thyroid's inability to produce enough thyroid hormone results in a slower metabolism. Weight gain is often one of the first symptoms, according to Mayo Clinic.
Along with weight gain, other symptoms of hypothyroidism include fatigue, weak muscles, stiff joints and constipation. As the condition worsens, the thyroid gland can become enlarged, referred to as goiter, as reported by Mayo Clinic.
Synthetic thyroid hormones treat the symptoms of hypothyroidism. Patients report some relief during the first week. After a couple of months of therapy, their symptoms are usually gone completely, as reported by WebMD.