The correct option is (c) 2 and 3 only.
Explanation
Iodine is an essential trace element required for the synthesis of thyroid hormones, specifically thyroxine (T4) and triiodothyronine (T3), by the thyroid gland. These hormones play a critical role in regulating basal metabolic rate, growth, and development in animals.
Statement-wise Analysis
- Statement 1 is Incorrect. Insulin is a peptide hormone produced by the beta cells of the Islets of Langerhans in the pancreas. It regulates blood glucose levels. The synthesis and structure of insulin do not require iodine; however, zinc is often associated with the storage of insulin in the pancreas. Iodine is exclusively required for thyroid hormones.
- Statement 2 is Correct. In amphibians, the process of metamorphosis (transformation from a larval tadpole to an adult frog) is initiated and regulated by the hormone thyroxine. Since iodine is a necessary component of thyroxine, its presence in the water is crucial. If the water is iodine-deficient, the thyroid gland of the tadpole cannot produce sufficient thyroxine, causing the tadpole to remain in the larval stage and fail to become an adult frog.
- Statement 3 is Correct. In humans, a deficiency of iodine in the diet leads to insufficient production of thyroid hormones. In response, the anterior pituitary gland secretes more Thyroid Stimulating Hormone (TSH) to stimulate the thyroid. This constant stimulation causes the thyroid gland to enlarge in an attempt to trap more iodine from the blood, resulting in a condition known as simple goitre.
Key Takeaway: Iodine is indispensable for the synthesis of thyroid hormones (thyroxine), which control metamorphosis in amphibians and prevent goitre in humans. It is not involved in the production of insulin.