Which of the following hormones does not act by a second messenger system ?

Options

A. Glucagon
B. Epinephrine
C. FSH
D. Testosterone

Answer

To determine which hormone does not act by a second messenger system, we need to understand the difference between hormones that utilize second messenger systems and those that do not.

### Step-by-Step Solution:

1. **Understand Hormones**: Hormones are chemical messengers secreted by glands in the endocrine system. They can be classified into two main categories: protein (or peptide) hormones and steroid hormones.

2. **Identify Hormone Types**:
- **Protein Hormones**: These are typically water-soluble and act on cell surface receptors. They require a second messenger system to relay their signal inside the cell. Examples include:
- Follicle Stimulating Hormone (FSH)
- Insulin
- Glucagon
- Growth Hormone
- **Steroid Hormones**: These are lipid-soluble and can pass through the cell membrane to bind to intracellular receptors. They do not require a second messenger system. Examples include:
- Testosterone
- Estrogen
- Progesterone

3. **Identify the Mechanism of Action**:
- **Second Messenger System**: This system involves the activation of a secondary messenger (like cyclic AMP, calcium ions, or inositol triphosphate) that transmits the signal inside the cell after the hormone binds to its receptor on the cell surface.
- **Direct Action**: Steroid hormones, on the other hand, bind to receptors inside the cell and directly influence gene expression without the need for a secondary messenger.

4. **Evaluate the Options**: The question asks for the hormone that does not act through a second messenger system. Based on the classifications:
- If the hormone is a steroid hormone, it does not use a second messenger system.
- If the hormone is a protein hormone, it does use a second messenger system.

5. **Conclusion**: The hormone that does not act by a second messenger system is a steroid hormone. For example, Testosterone is a steroid hormone and acts directly without the need for a second messenger.

### Final Answer:
The hormone that does not act by a second messenger system is **Testosterone**.