**Core Concept**
Hypothyroidism is characterized by an underproduction of thyroid hormones, leading to various clinical manifestations. The condition arises due to a deficiency in thyroid hormone production, which can be caused by primary, secondary, or tertiary hypothyroidism. The primary causes include autoimmune thyroiditis (Hashimoto's disease), thyroid surgery, or radioactive iodine ablation.

**Why the Correct Answer is Right**
The correct answer is related to the clinical presentation of hyperthyroidism, which is characterized by an overproduction of thyroid hormones. The symptoms include weight loss, palpitations, heat intolerance, tremors, exophthalmos, and nervousness. The increased metabolic rate leads to an increased heart rate, cardiac output, and peripheral vasodilation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Typically seen in hyperthyroidism, exophthalmos is caused by the autoimmune reaction against the extraocular muscles and surrounding tissues.
* **Option B:** Weight loss is a common feature of hyperthyroidism due to increased metabolic rate and heat intolerance.
* **Option C:** Nervousness is a characteristic feature of hyperthyroidism due to increased sympathetic activity and elevated circulating catecholamines.

**Clinical Pearl / High-Yield Fact**
Hyperthyroidism can be caused by various factors, including Graves' disease, toxic multinodular goiter, and thyrotoxic crisis. It is essential to differentiate between these causes and manage the condition promptly to prevent long-term complications.

**Correct Answer: B. Weight gain is a characteristic feature of hypothyroidism, not hyperthyroidism.**