Final answer:
Low TSH levels are generally indicative of hyperthyroidism, resulting in weight loss, not weight gain. It is symptomatic of an overactive thyroid leading to an increased metabolic rate and an increased heart rate.
Step-by-step explanation:
Low thyroid-stimulating hormone (TSH) levels typically indicate hyperthyroidism, which is an overproduction of thyroid hormones. This results in an increased metabolic rate, leading to symptoms such as weight loss, heat intolerance, profuse sweating, and an increased heart rate. Contrary to a low TSH leading to weight gain, it generally causes weight loss. To address the confusion in the options given, if a person has low TSH, it's typically because their thyroid gland is overactive, producing excessive thyroid hormones (T3 and T4), and the pituitary gland reduces the output of TSH in response to the high levels of thyroid hormones. However, a high TSH level is associated with hypothyroidism which can lead to symptoms of weight gain and fatigue, because high TSH indicates a struggling thyroid that is not producing enough hormones, prompting the pituitary gland to produce more TSH to stimulate the thyroid.