The main hormone secreted by the Thyroid gland

The correct option is (a) and (b) Both (which refers to T4 and T3).

To understand why this is the correct answer, let's break down the hormones produced and secreted by the thyroid gland:
1. Thyroxine (T4): Also known as tetraiodothyronine, T4 is the primary hormone secreted by the thyroid gland. It represents approximately 90% of the active thyroid hormones released into the bloodstream.
2. Triiodothyronine (T3): T3 is the biologically active form of the thyroid hormone, representing about 10% of the thyroid's secretions. While much of the body's T3 is created by converting T4 in peripheral tissues, the thyroid gland itself directly secretes a portion of it.
3. Thyroid-Stimulating Hormone (TSH): TSH is not secreted by the thyroid gland. Instead, it is produced and secreted by the anterior pituitary gland to stimulate the thyroid gland to produce T3 and T4.

Therefore, because the thyroid gland directly secretes both T4 (a) and T3 (b), the correct answer is "(a) and (b) Both".