The correct option is Both A and R are true and R is the correct explanation of A.
Explanation
The question addresses the physiological relationship between dietary minerals and endocrine function, specifically focusing on the thyroid gland and its regulation.
Analysis of Assertion (A):
Correct. The thyroid gland enlarges when there is a deficiency of iodine in the diet. This enlargement is a compensatory mechanism where the gland increases in size (hypertrophy) to maximize the uptake of available iodine from the blood. This condition is clinically known as simple goitre.
Analysis of Reason (R):
Correct. Iodine is a critical raw material required for the synthesis of thyroid hormones, specifically thyroxin (tetraiodothyronine or T4) and triiodothyronine (T3). These hormones are responsible for regulating the basal metabolic rate (BMR) and controlling the metabolism of carbohydrates, proteins, and fats in the body.
Establishment of Cause and Effect:
Because iodine is essential for thyroxin synthesis (R), its deficiency leads to low levels of thyroid hormones in the blood. Low thyroxin levels trigger the anterior pituitary gland to release excess Thyroid Stimulating Hormone (TSH). This continuous stimulation causes the thyroid follicles to proliferate and the gland to enlarge (A). Therefore, the physiological role of iodine described in R explains the pathology described in A.
Key Takeaway:
Iodine is indispensable for the synthesis of thyroxin. A deficiency disrupts the negative feedback loop of the pituitary-thyroid axis, leading to elevated TSH levels and subsequent thyroid enlargement (goitre).