Which of the following stimulates the release of parathyroid hormone?

Correct Answer: D

Rationale: The correct answer is D: low levels of calcium in the blood. Parathyroid hormone is released in response to low blood calcium levels to help increase calcium levels by promoting its release from bones and enhancing its absorption in the intestines. This response helps maintain calcium homeostasis. TSH from the posterior pituitary gland (choice A) is not related to parathyroid hormone release. High levels of calcium in the blood (choice B) would not stimulate parathyroid hormone release as it is released in response to low calcium levels. Calcitonin from the anterior pituitary gland (choice C) actually works in opposition to parathyroid hormone by promoting calcium deposition in bones and decreasing blood calcium levels.

Correct Answer: D

Rationale: The correct answer is D because peptide hormones are hydrophilic molecules that cannot pass through the cell membrane, so they bind to receptors on the cell surface to initiate signaling pathways within the cell. Therefore, they are unable to enter the target cell like lipid-soluble hormones do. By binding to specific receptors on the cell surface, peptide hormones trigger a series of events that ultimately lead to a cellular response. Therefore, all of the given statements are true, making D the correct choice.

Correct Answer: C

Rationale: The correct answer is C: Tropic hormones stimulate other endocrine glands to secrete hormones. Tropic hormones are released from the anterior pituitary gland and act on target glands to regulate the secretion of specific hormones. They help maintain homeostasis by controlling hormone levels in the body. Choices A and B are incorrect because tropic hormones do not target the pineal gland or thymus gland. Choice D is incorrect because tropic hormones primarily target endocrine glands, not nervous tissue. In summary, tropic hormones play a key role in regulating hormone production in other endocrine glands, making choice C the correct answer.

Correct Answer: D

Rationale: The correct answer is D because hormone secretion is regulated by various factors. The nervous system can directly stimulate or inhibit hormone release. Other hormones can act as signals to trigger or suppress the secretion of specific hormones. Changes in blood composition, such as glucose levels or electrolyte concentrations, can also influence hormone production and release. Therefore, all of the options play a role in regulating hormone secretion, making choice D the correct answer.

Correct Answer: D

Rationale: The correct answer is D: second messengers. Protein or amino acid-based hormones are unable to pass through the cell membrane due to their large size and hydrophilic nature. Therefore, they bind to specific receptors on the cell surface, triggering a series of events that lead to the production of second messengers inside the cell. These second messengers, such as cyclic AMP or calcium ions, then transmit the hormonal signal to the target cell's interior to initiate a physiological response. Choices A, B, and C are incorrect because protein or amino acid-based hormones do not directly exert their effects through ions, deactivators, or nucleotides intracellularly.