Effects of thyroxine on metabolic rate is:

Options

A. Decreases
B. No effect
C. Increases
D. Uncertain

Answer

### Step-by-Step Solution:

1. **Understanding Thyroxine**: Thyroxine, also known as T4 or tetraiodothyronine, is a hormone produced by the thyroid gland. It plays a crucial role in regulating metabolism in the body.

2. **Location of the Thyroid Gland**: The thyroid gland is located on either side of the trachea (windpipe) in the neck. It releases hormones that influence metabolic processes.

3. **Hormones Involved**: The primary hormones released by the thyroid gland are T3 (triiodothyronine) and T4 (thyroxine). Both of these hormones are essential for regulating the basal metabolic rate (BMR).

4. **Effects of Thyroxine on Metabolic Rate**:
- **Increased Metabolic Rate**: Thyroxine increases the basal metabolic rate. This means that when thyroxine levels are high, the body burns more calories at rest.
- **Hyperthyroidism**: In conditions where there is an overproduction of thyroid hormones (hyperthyroidism), the metabolic rate is significantly increased. This can lead to symptoms such as weight loss, increased heart rate, and heightened energy levels.
- **Hypothyroidism**: Conversely, in conditions where there is a deficiency of thyroid hormones (hypothyroidism), the metabolic rate decreases, leading to weight gain, fatigue, and reduced energy levels.

5. **Conclusion**: Based on the effects of thyroxine on metabolic rate, we conclude that thyroxine increases the metabolic rate. Therefore, the correct answer to the question is that the effect of thyroxine on metabolic rate is an increase.

### Final Answer:
The effect of thyroxine on metabolic rate is an increase.