Blood levels of which of the following substances is decreased in Graves disease?

Correct Answer: D

Rationale: In Graves disease, there is an overproduction of thyroid hormones due to autoimmune stimulation. Thyroid-stimulating hormone (TSH) is decreased because the excessive thyroid hormones suppress its production through negative feedback. A and B are incorrect because both T3 and T4 levels are elevated in Graves disease. C is incorrect as DIT is a precursor to thyroid hormones and its levels may not necessarily be decreased in Graves disease.

Correct Answer: B

Rationale: The correct answer is B because the parathyroid glands target the bone and activate osteoclasts to release calcium into the bloodstream. This process helps maintain adequate blood calcium levels. By activating osteoclasts, calcium is released from the bone into the blood, ensuring a balance of calcium levels. A: Blocking the action of growth hormone is unrelated to the function of parathyroid glands in maintaining blood calcium levels. C: Antagonizing the synthesis of calcitonin is not the primary mechanism through which parathyroid glands regulate blood calcium levels. D: Slowing the activity of tissues that require calcium for activity would not help maintain adequate blood calcium levels as it does not address the direct release of calcium from bone by activating osteoclasts.

Correct Answer: A

Rationale: The correct answer is A: TH (Thyroid Hormone). TH is the only amine hormone that acts like a steroid because it is derived from an amino acid but functions similarly to steroid hormones by entering target cells and binding to intracellular receptors to regulate gene expression. ACTH, GH, and ADH are peptide hormones that do not act like steroids. ACTH stimulates the adrenal glands to produce cortisol, GH stimulates growth, and ADH regulates water balance in the body.

Correct Answer: C

Rationale: The correct answer is C because peptide hormones typically bind to receptor proteins on the plasma membrane. This is because peptide hormones are not able to pass through the cell membrane due to their size and polarity. Once the hormone binds to the receptor on the plasma membrane, it activates a second messenger system within the cell, leading to various cellular responses. Choice A is incorrect because peptide hormones do not bind directly to intracellular components like second messengers inside the cell. Choice B is incorrect because peptide hormones do not directly activate particular genes within the cell. Choice D is incorrect because peptide hormones do not bind directly to genes on the plasma membrane.

Correct Answer: C

Rationale: The correct answer is C because ADH (antidiuretic hormone) increases water reabsorption by the kidneys (increasing blood volume), causes vasoconstriction (narrowing of blood vessels), and ultimately leads to an increase in blood pressure. Choice A is incorrect because ADH does not decrease water reabsorption. Choice B is incorrect because ADH does not cause dilation of blood vessels, which would decrease blood pressure. Choice D is incorrect because ADH does not cause dilation of blood vessels, which would not lead to a decrease in blood pressure.