**Question:** A 29-year-old woman complains of nervousness and muscle weakness of 6 months in duration. She is intolerant of heat and sweats excessively. She has lost 9 kg (20 lb pounds) over past 6 months, despite increased caloric intake. She frequently finds her heart racing and can feel it pounding in her chest. She also states that she has missed several menstrual periods over the past few months. Physical examination reveals warm and moist skin and bulging eyes (exophthalmos). Laboratory studies will likely reveal which of the following endocrine abnormalities in this patient?

A. Hyperthyroidism
B. Hypothyroidism
C. Cushing's syndrome
D. Diabetes mellitus

**Answer:**

**Core Concept:** The described symptoms and signs in this patient are indicative of an endocrine disorder, specifically affecting the thyroid gland, adrenal glands, or pituitary gland. These symptoms and signs include:

1. Nervousness and muscle weakness: These are common symptoms of thyroid disorders, particularly hyperthyroidism, which is characterized by overactive thyroid gland leading to increased thyroid hormone levels in the bloodstream.
2. Heat intolerance and excessive sweating: This is a common feature of hyperthyroidism due to increased thyroid hormone levels.
3. Weight loss: Inadequate thyroid hormone production (hypothyroidism) would lead to weight gain, whereas increased thyroid hormone levels cause weight loss, as seen in this patient.
4. Palpitations and chest pain: Increased heart rate occurs in hyperthyroidism due to higher thyroid hormone levels.
5. Bulging eyes (exophthalmos) and warm skin: These symptoms are indicative of Cushing's syndrome, which is caused by excess cortisol production, usually due to an adrenal gland tumor.
6. Missed menstrual periods: The primary cause of irregular menstrual cycles in this patient is likely Cushing's syndrome, which affects the pituitary gland and leads to a decrease in sex hormone production.

**Why the Correct Answer is Hyperthyroidism:**

The correct answer, **Hyperthyroidism**, is chosen due to the following reasons:

1. **Option A (Hyperthyroidism):** The described symptoms and signs are consistent with increased thyroid hormone levels in the bloodstream, which is characteristic of hyperthyroidism.
2. **Option B (Hypothyroidism):** Hypothyroidism would lead to weight gain, not weight loss. Moreover, hypothyroidism does not cause palpitations or chest pain due to increased heart rate.
3. **Option C (Cushing's syndrome):** Cushing's syndrome causes weight gain, bloating, and moon-shaped face, which are not present in this patient. Additionally, Cushing's syndrome is characterized by suppressed sex hormone production, which does not explain the missed menstrual periods in this patient.
4. **Option D (Diabetes Mellitus):** Diabetes mellitus causes hyperglycemia, polyuria, polydipsia, and polyphagia, none of which are present in this patient. Moreover, diabetes mellitus does not cause pal