**Core Concept:** Hirsutism and voice change are clinical manifestations that can be associated with hormonal imbalances, particularly excess androgen production.

**Why the Correct Answer is Right:** In this case, the best test to be done would be measuring serum testosterone levels (Option D) as it directly assesses the cause of the patient's symptoms.

**Why Each Wrong Option is Incorrect:**

A. Thyroid function tests (TFTs): While TFTs are important in evaluating thyroid disorders, they do not specifically address the issue of increased androgen production.

B. Serum estradiol and progesterone levels: These tests are related to female sex steroids, but they are not the primary cause of hirsutism and voice change in this scenario.

C. Pelvic ultrasound: This test is useful for evaluating pelvic pathology but does not assess hormonal disturbances or directly diagnose the cause of the symptoms.

**Clinical Pearl:**

Hirsutism can be caused by various factors, including polycystic ovary syndrome (PCOS), Cushing's syndrome, and certain medications. Understanding the etiology and appropriate investigations will help guide further management.

**Correct Answer:** D. Serum testosterone levels