Final answer:
The palpable goiter in the normal rat was due to increased thyroid hormone synthesis stimulated by TSH injection, which, in the case of iodine deficiency, leads to overproduction of TSH without sufficient thyroid hormone formation and goiter development.
Step-by-step explanation:
The normal rat developed a palpable goiter with the TSH (Thyroid Stimulating Hormone) injection as a result of increased thyroid hormone synthesis. This is due to the fact that when TSH is injected, it stimulates the thyroid gland to produce more thyroid hormones (T3 and T4). If there is an adequate supply of iodine, the thyroid will produce more hormones to meet the body's demands. However, in the case of an iodine deficiency, the thyroid cannot synthesize the necessary hormones, even with extra stimulation by TSH. This leads to an overproduction of TSH without the formation of thyroid hormone and subsequently, higher levels of colloid accumulation in the thyroid, which enlarges and forms a goiter.