The hormone primarily responsible for lowering serum calcium concentration is Calcitonin.

Calcium homeostasis in the body is tightly regulated by several hormones, primarily Parathyroid hormone (PTH), Calcitonin, and Vitamin D (Calcitriol). Among these:

- Parathyroid hormone (PTH) acts to increase serum calcium levels by stimulating bone resorption, increasing calcium reabsorption in the kidneys, and promoting activation of Vitamin D to increase intestinal calcium absorption.
- Vitamin D (Calcitriol) enhances intestinal absorption of calcium and phosphate, thereby indirectly increasing serum calcium levels.
- Calcitonin, secreted by the thyroid gland's parafollicular cells (C cells), acts to lower serum calcium concentration. It does this by inhibiting osteoclastic bone resorption, which reduces the release of calcium from bones into the bloodstream. Although calcitonin's role is less prominent in humans compared to PTH, it remains the primary hormone responsible for lowering high serum calcium levels.
- Thyroxine primarily regulates metabolism and does not play a direct role in calcium homeostasis.

Therefore, the correct answer is Calcitonin.

Reference: Ganong's Review of Medical Physiology, 26th Edition, Chapter 22: Calcium and Phosphate Metabolism