Thyroid hormone exerts its influence by ________.

Correct Answer: A

Rationale: The correct answer is A because thyroid hormone is a lipid-soluble hormone that can diffuse across the cell membrane and bind to intracellular receptors within the nuclei. This binding activates gene transcription, leading to various metabolic effects. Choice B is incorrect because thyroid hormone actually has a significant impact on body metabolism. Choice C is incorrect as thyroid hormone does not directly affect blood vessel adrenergic receptors or blood pressure. Choice D is incorrect as thyroid hormone actually acts to increase basal metabolic rate, not decrease it.

Correct Answer: D

Rationale: The correct answer is D: Parathyroid hormone is the single most important regulator of calcium levels in the blood. Parathyroid hormone plays a crucial role in maintaining calcium homeostasis by stimulating bone resorption, increasing calcium reabsorption in the kidneys, and activating vitamin D to enhance calcium absorption in the intestines. It acts to raise blood calcium levels when they are low. Explanation for other choices: A: Increased calcitonin levels actually lower blood calcium levels by promoting calcium deposition in bones. B: High calcium levels do not cause bone resorption; it is actually the opposite as mentioned in choice D. C: Parathyroid hormone does not increase osteoblast activity; it primarily affects osteoclasts to release calcium from bones.

Correct Answer: D

Rationale: The correct answer is D: breaking down nutrients. Hormones are chemical messengers that regulate various physiological processes in the body. Hormones do not directly break down nutrients; instead, they regulate the processes involved in nutrient breakdown. A: maintaining homeostasis is a function of hormones as they help regulate internal balance. B: influencing metabolism is a function of hormones as they regulate energy production and utilization. C: influencing growth and development is a function of hormones as they play a crucial role in the growth and maturation of cells and tissues.

Correct Answer: B

Rationale: The correct answer is B: kidney and blood vessels. ADH, or antidiuretic hormone, acts on the kidney to regulate water reabsorption and maintain fluid balance in the body. It also acts on blood vessels to regulate blood pressure. The other choices are incorrect because ADH does not target the adrenal cortex, liver, pituitary gland, or thyroid gland specifically for its primary functions. ADH primarily acts on the kidney and blood vessels to regulate water balance and blood pressure in the body.

Correct Answer: C

Rationale: The correct answer is C: pituitary dwarfism. Growth hormone deficiency during growing years results in pituitary dwarfism due to stunted growth. Myxedema (choice A) is associated with hypothyroidism, not growth hormone deficiency. Gigantism (choice B) occurs due to excessive growth hormone production in childhood. Acromegaly (choice D) is caused by excess growth hormone in adulthood, not during the growing years. Thus, choice C is the most appropriate based on the impact of growth hormone deficiency on growth during childhood.