This condition of chronic inflammation of the thyroid leading to under-activity is

The correct option is Thyroiditis.

Here is a step-by-step breakdown of why this option is correct:
1. Understanding the Terminology: In medical terminology, the suffix "-itis" denotes inflammation. Therefore, inflammation of the thyroid gland is referred to as thyroiditis.
2. Chronic Inflammation and Under-activity: Chronic thyroiditis (most commonly Hashimoto's thyroiditis) is an autoimmune condition where the body's immune system slowly destroys the thyroid gland. As this chronic inflammation progresses, the thyroid gland loses its ability to produce sufficient thyroid hormones, eventually leading to under-activity (hypothyroidism). Thus, the underlying inflammatory condition that leads to this under-activity is thyroiditis.
3. Evaluating the Other Options:
- Goitre: This refers to any abnormal enlargement of the thyroid gland, which can occur with under-activity, over-activity, or even normal activity.
- Hypothyroidism: This is the clinical state of thyroid under-activity itself (the outcome), rather than the specific condition of chronic inflammation that causes it.
- Hyperthyroidism: This is the condition of thyroid over-activity, which is the opposite of under-activity.