**Core Concept**
Thyroid-stimulating hormone (TSH) is a hormone secreted by the pituitary gland that regulates the production of thyroid hormones (T3 and T4) by the thyroid gland. A normal TSH level indicates that the thyroid gland is functioning within a normal range, producing adequate amounts of T3 and T4.

**Why the Correct Answer is Right**
In a euthyroid state, the pituitary gland maintains a feedback loop with the thyroid gland to regulate thyroid hormone production. When thyroid hormone levels are normal, the hypothalamus secretes thyrotropin-releasing hormone (TRH), which stimulates the pituitary gland to release TSH. The TSH then stimulates the thyroid gland to produce T3 and T4. A normal TSH level, therefore, suggests that the thyroid gland is producing thyroid hormones at a normal rate.

**Why Each Wrong Option is Incorrect**
**Option A:** In primary hypothyroidism, the thyroid gland is underactive, producing inadequate amounts of T3 and T4. As a result, TSH levels are elevated (not normal) to stimulate the thyroid gland to produce more hormones.

**Option B:** In primary hyperthyroidism, the thyroid gland is overactive, producing excessive amounts of T3 and T4. As a result, TSH levels are suppressed (not normal) due to negative feedback from the high levels of thyroid hormones.

**Option C:** In secondary hypothyroidism, the pituitary gland is underactive, leading to inadequate secretion of TSH. As a result, thyroid hormone production is reduced, and TSH levels are low (not normal).

**Option D:** In thyroiditis, the thyroid gland is inflamed, leading to a temporary imbalance in thyroid hormone production. While TSH levels may be normal initially, they often become elevated or suppressed as the inflammation resolves and the thyroid gland returns to its normal function.

**Clinical Pearl / High-Yield Fact**
A normal TSH level is essential for diagnosing hypothyroidism and hyperthyroidism, as it helps to differentiate between primary and secondary causes of thyroid dysfunction.

**Correct Answer: B. Primary hyperthyroidism suppresses TSH levels due to negative feedback from high levels of thyroid hormones.**