Thyroxin is a hormone secreted by the thyroid gland and plays a crucial role in regulating the metabolism of the body. Over-secretion of thyroxin, a condition known as hyperthyroidism, can cause several health problems. One of the most common symptoms of hyperthyroidism is weight loss or thinness in the body, even with an increased appetite. This is because the excess thyroxin in the body increases the rate of metabolism, leading to increased energy consumption and weight loss. Other symptoms of hyperthyroidism may include an increased heart rate, sweating, irritability, and nervousness. Therefore, the correct option from the given choices is "thinness in body."