A 42-year-old woman has noticed dry skin, fatigue, and weight gain over the past 3 months. Her blood pressure is 110/70 mm Hg, pulse 60/min, and heart and lungs are normal. Her skin feels rough and dry, but the rest of the examination is normal. Her biochemistry is normal but the thyroid-stimulating hormone (TSH) is 39 mU/L (0.5-5 mU/L). Which of the following is the most likely cause for her elevated TSH?

**Core Concept**
The patient's elevated TSH level suggests primary hypothyroidism, a condition where the thyroid gland fails to produce sufficient thyroid hormones. This is often due to autoimmune thyroiditis, where the immune system attacks the thyroid gland.

**Why the Correct Answer is Right**
In autoimmune thyroiditis, specifically Hashimoto's thyroiditis, the immune system produces antibodies that target the thyroid gland, leading to inflammation and damage. This results in a decrease in thyroid hormone production, causing an increase in TSH levels as the pituitary gland attempts to stimulate the thyroid gland to produce more hormones. The patient's symptoms, such as dry skin, fatigue, and weight gain, are classic signs of hypothyroidism.

**Why Each Wrong Option is Incorrect**
**Option A:** "Pituitary gland tumor" is incorrect because while a pituitary tumor could cause an increase in TSH, it would be an isolated finding and not typically associated with the patient's symptoms. Additionally, a pituitary tumor would more likely cause an overproduction of thyroid hormones, not a decrease.

**Option B:** "Thyroiditis due to radiation exposure" is incorrect because while radiation exposure can cause thyroiditis, it is less likely to cause an isolated increase in TSH without other symptoms. Additionally, radiation-induced thyroiditis typically presents with a more acute onset.

**Option C:** "Thyroid hormone resistance" is incorrect because thyroid hormone resistance typically presents with an increase in TSH levels, but the patient's symptoms and biochemistry are more consistent with hypothyroidism, not resistance.

**Clinical Pearl / High-Yield Fact**
Autoimmune thyroiditis, specifically Hashimoto's thyroiditis, is the most common cause of primary hypothyroidism in women, often presenting with nonspecific symptoms such as dry skin, fatigue, and weight gain.

**Correct Answer:** D. Hashimoto's thyroiditis

✓ Correct Answer: C. primary hypothyroidism