**Core Concept**
Thyrotoxicosis is a condition characterized by the presence of excess thyroid hormones in the body, leading to various systemic effects. This condition can arise from different causes, including Graves' disease, thyroiditis, and thyroid nodules. The clinical manifestations of thyrotoxicosis are diverse and can affect multiple organ systems.

**Why the Correct Answer is Right**
The correct answer is related to a condition that is not typically seen in thyrotoxicosis. In thyrotoxicosis, the increased levels of thyroid hormones stimulate the body's metabolic rate, leading to weight loss, heat intolerance, and other symptoms. One of the key features of thyrotoxicosis is the presence of tachycardia, or an elevated heart rate, due to the increased metabolic demand. Additionally, patients with thyrotoxicosis may exhibit tremors, muscle weakness, and other signs of hypermetabolism.

**Option A:** Muscle hypertrophy
* Muscle hypertrophy is not typically seen in thyrotoxicosis, where muscle atrophy is more common due to the catabolic effects of excess thyroid hormones.

**Option B:** Tachypnea
* Tachypnea, or rapid breathing, can be seen in thyrotoxicosis due to the increased metabolic rate and respiratory drive.

**Option C:** Weight gain
* Weight gain is not typically seen in thyrotoxicosis, where weight loss is a common symptom due to the increased metabolic rate.

**Option D:** Hyperreflexia
* Hyperreflexia, or increased reflexes, can be seen in thyrotoxicosis due to the increased sympathetic activity and peripheral nerve stimulation.

**Clinical Pearl / High-Yield Fact**
In thyrotoxicosis, the presence of tachycardia, tremors, and muscle weakness is a hallmark of the condition. These symptoms are due to the increased metabolic rate and catabolic effects of excess thyroid hormones.

**Correct Answer:** C. Weight gain