Unexplained weight gain or weight loss is not always related to diet or lifestyle. One possible factor is a thyroid disorder. However, weight changes alone cannot confirm thyroid disease because similar symptoms may occur with many other health conditions.

The thyroid is a small, butterfly-shaped gland located at the front of the neck. It produces the hormones thyroxine (T4) and triiodothyronine (T3), which help regulate how the body uses energy. Thyroid hormones therefore influence many body functions, including metabolism, heart rate, body temperature, digestion, and body weight.

When Thyroid Hormone Levels Are Too High

Hyperthyroidism occurs when the thyroid gland produces more thyroid hormone than the body needs. Symptoms may include unexplained weight loss, a rapid or irregular heartbeat, shaky hands, increased sweating, difficulty tolerating heat, trouble sleeping, and more frequent bowel movements.

One common cause of hyperthyroidism is Graves’ disease, an autoimmune disorder in which the immune system stimulates the thyroid to produce excessive amounts of hormone. Graves’ disease may also affect the eyes. Some people can develop redness or irritation, double vision, or protruding eyes due to inflammation of the tissues around the eyes.

When Thyroid Hormone Levels Are Too Low

Hypothyroidism occurs when the body does not have enough thyroid hormone. Symptoms often develop gradually and can vary from person to person. Common signs include fatigue, weight gain, increased sensitivity to cold, dry skin, dry or thinning hair, a slower heart rate, and constipation.

Symptoms Alone Are Not Enough for a Diagnosis

Many symptoms of both hyperthyroidism and hypothyroidism overlap with those of other medical conditions. For this reason, thyroid disorders cannot be diagnosed based on symptoms alone. A doctor may review the patient's medical history, perform a physical examination, and order laboratory tests when appropriate.

A TSH (thyroid-stimulating hormone) test is commonly used as an initial test to evaluate thyroid function. Depending on the results and clinical findings, additional tests such as free T4, T3, or thyroid antibody tests may be recommended. A high TSH level is most commonly associated with an underactive thyroid, while a low TSH level may indicate an overactive thyroid, although results should always be interpreted together with other clinical and laboratory findings.

If unexplained weight changes are accompanied by symptoms such as palpitations, excessive sweating, hand tremors, fatigue, intolerance to heat or cold, dry skin, or changes in bowel habits, medical evaluation can help identify the underlying cause and determine the appropriate next steps.