**Core Concept**
Thyrotoxicosis is a clinical syndrome resulting from excessive circulating levels of thyroid hormones, primarily triiodothyronine (T3) and thyroxine (T4). This condition can be caused by various underlying pathologies, including Graves' disease, toxic multinodular goiter, and thyroiditis.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of thyrotoxicosis. In this condition, the increased levels of thyroid hormones lead to a decrease in the peripheral conversion of T4 to T3. However, the correct answer is not about peripheral conversion. Instead, it is about the effect of thyrotoxicosis on the body's metabolic rate. Thyrotoxicosis increases the metabolic rate, leading to an increase in oxygen consumption, heart rate, and a decrease in body temperature.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not directly related to the pathophysiology of thyrotoxicosis.
**Option B:** This option is incorrect because it is not a characteristic of thyrotoxicosis.
**Option C:** This option is incorrect because it is a characteristic of hypothyroidism, not thyrotoxicosis.

**Clinical Pearl / High-Yield Fact**
In thyrotoxicosis, the increased metabolic rate can lead to a decrease in body temperature, which can be a distinguishing feature from other conditions. This decrease in body temperature can be a useful clinical clue in diagnosing thyrotoxicosis.

**Correct Answer:** D.