Final answer:
The likely cause of the symptoms after thyroid gland removal is the accidental removal of the parathyroid gland, resulting in hypoparathyroidism and low calcium levels.
Step-by-step explanation:
The likely cause of the patient's symptoms of weakness, muscle spasms, and low heart rate (bradycardia) after surgery to remove the thyroid gland is the accidental removal of the parathyroid gland. The parathyroid gland is responsible for producing parathyroid hormone (PTH), which helps regulate calcium levels in the blood. Removal of the parathyroid gland can lead to hypoparathyroidism, resulting in low calcium levels and neuro-muscular irritability.