In a thyroidectomized animal (thyroid gland removed), which treatment best restores normal thyroid hormone signaling and negative feedback on pituitary TSH secretion?

• A. Administer exogenous TSH to stimulate thyroid hormone production.
• B. Administer a thyroid peroxidase inhibitor (e.g., methimazole) to prevent hormone loss.
• C. Administer exogenous levothyroxine (T4) to replace missing thyroid hormone.
• D. Administer iodine supplementation alone to support thyroid hormone synthesis.

Answer: (C)

Verified step by step guidance

1. Understand the physiological role of the thyroid gland: it produces thyroid hormones (mainly T4 and T3) that regulate metabolism and provide negative feedback to the pituitary gland to control TSH secretion.
2. Recognize that in a thyroidectomized animal, the thyroid gland is removed, so endogenous production of thyroid hormones (T4 and T3) is absent.
3. Evaluate the options: administering exogenous TSH would not be effective because there is no thyroid gland to respond and produce hormones; administering a thyroid peroxidase inhibitor would further block hormone synthesis, which is counterproductive; iodine supplementation alone is insufficient without a functioning thyroid gland.
4. Identify that the best treatment is to administer exogenous thyroid hormone (levothyroxine, T4) to replace the missing hormones, thereby restoring normal hormone signaling and re-establishing negative feedback on pituitary TSH secretion.
5. Conclude that exogenous T4 administration mimics natural hormone levels, suppresses excess TSH secretion via negative feedback, and maintains metabolic homeostasis.