**Core Concept**
Addison's disease is a rare endocrine disorder characterized by the insufficient production of cortisol and aldosterone hormones by the adrenal glands. This condition is often caused by autoimmune destruction of the adrenal cortex, which can lead to a deficiency in cortisol and aldosterone production.

**Why the Correct Answer is Right**
Addison's disease is associated with several clinical features, including weight loss, fatigue, hypotension, and hyperpigmentation. The hyperpigmentation is due to the increased production of melanocyte-stimulating hormone (MSH) by the pituitary gland in response to the low cortisol levels. The adrenal glands produce cortisol, which is essential for the regulation of the immune response, metabolism, and response to stress. Aldosterone, another hormone produced by the adrenal glands, is crucial for maintaining electrolyte balance and blood pressure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Addison's disease is not directly associated with diabetes mellitus. However, patients with Addison's disease may have an increased susceptibility to infections and may develop secondary infections, which can lead to a hyperglycemic state, but this is not a direct association.
**Option B:** This option is incorrect because Addison's disease is not typically associated with an increased risk of osteoporosis. In fact, patients with Addison's disease often have an increased risk of osteoporosis due to the deficiency of cortisol, which plays a role in regulating bone metabolism.
**Option C:** This option is incorrect because Addison's disease is not directly associated with an increased risk of thyroiditis. However, patients with Addison's disease may have an increased susceptibility to autoimmune disorders, including thyroiditis, due to the underlying autoimmune nature of the disease.

**Clinical Pearl / High-Yield Fact**
One key clinical feature of Addison's disease is the presence of hyperpigmentation, which is due to the increased production of MSH by the pituitary gland. This can manifest as darkening of the skin, particularly in areas exposed to the sun.

**Correct Answer: D.**