Concept:The thyroid gland mainly produces and releases the hormone thyroxine (T4), which regulates body metabolism, growth, and development.
Explanation:The thyroid gland uses iodine to make two key hormones: triiodothyronine (T3) and thyroxine (T4).
Thyroxine is the primary hormone secreted by the thyroid into the bloodstream.
The hypothalamus and pituitary control thyroid hormone release via TRH and TSH.
Insulin is made by the pancreas, not the thyroid.
Parathykalin is not a thyroid hormone; the parathyroid gland produces parathyroid hormone.
Thus, the only correct option among the given choices is thyroxine.
Answer:A. thyroxine