In a laboratory experiment studying thyroid function, which rat(s) would be considered euthyroid without receiving any injections?

• A. The normal rat
• B. The thyroidectomized rat
• C. All rats in the experiment
• D. The hypophysectomized rat

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the term 'euthyroid' — it refers to an organism having normal thyroid function, meaning normal levels of thyroid hormones are present and the thyroid gland is functioning properly.
2. Step 2: Identify the conditions of each rat in the experiment: a normal rat has an intact thyroid gland and normal thyroid function; a thyroidectomized rat has had its thyroid gland removed, so it cannot produce thyroid hormones; a hypophysectomized rat has had its pituitary gland removed, which affects thyroid-stimulating hormone (TSH) production and thus thyroid function.
3. Step 3: Analyze which rat(s) would maintain normal thyroid hormone levels without any injections: the normal rat naturally produces thyroid hormones, while the thyroidectomized and hypophysectomized rats would have impaired thyroid hormone production without supplementation.
4. Step 4: Conclude that only the normal rat is euthyroid without receiving any injections, because it has an intact and functioning thyroid axis.
5. Step 5: Summarize that the thyroidectomized and hypophysectomized rats require hormone replacement or stimulation to achieve euthyroid status, so they are not euthyroid without injections.