Endocrinology Practice Questions

Q: Which of the following is a lipophilic hormone that acts on nuclear receptors, while the others are hydrophilic hormones that act on cytosolic receptors?

Thyroxine. ### Explanation The classification of hormones based on their chemical nature and receptor location is a high-yield topic for NEET-PG. **Correct Answer: A. Thyroxine** Thyroxine ($T_4$) and Triiodothyronine ($T_3$) are amine-derived hormones, but unlike other amines, they are highly **lipophilic** (lipid-soluble). This property allows them to cross the cell membrane via transporters and bind to **nuclear receptors** (specifically TR$\alpha$ and TR$\beta$). Once bound, they act as transcription factors to modulate gene expression. **Analysis of Incorrect Options:** * **B. Epinephrine:** Although also an amine (derived from tyrosine), epinephrine is **hydrophilic**. It cannot cross the lipid bilayer and must bind to **G-protein coupled receptors (GPCRs)** on the cell surface. * **C. Growth Hormone (GH):** This is a large peptide hormone. Being hydrophilic, it binds to **extracellular receptors** (specifically JAK-STAT linked receptors) on the plasma membrane. * **D. ACTH:** This is a polypeptide hormone. Like GH, it is water-soluble and acts via **cell surface receptors** (specifically MC2R, which uses the cAMP second messenger system). **High-Yield Clinical Pearls for NEET-PG:** * **Exceptions to the Rule:** Most amine hormones (Catecholamines) are hydrophilic, but **Thyroid hormones** are the notable lipophilic exception. * **Receptor Locations:** * **Cytosolic Receptors:** Primarily Steroid hormones (e.g., Cortisol, Aldosterone, Vitamin D). * **Nuclear Receptors:** Thyroid hormones ($T_3/T_4$), Retinoic acid, and Estrogen (partially). * **Speed of Action:** Surface-acting hormones (Epinephrine) act within seconds via second messengers, while nuclear-acting hormones (Thyroxine) take hours to days to show effects due to the time required for protein synthesis.

Q: Calcium absorption from the gut is enhanced by which of the following?

1,25-dihydroxycholecalciferol. **Explanation:** The correct answer is **1,25-dihydroxycholecalciferol (Calcitriol)**. **1. Why 1,25-dihydroxycholecalciferol is correct:** Calcitriol is the active form of Vitamin D. Its primary function is to increase plasma calcium levels by enhancing intestinal absorption. It acts on the enterocytes of the duodenum and jejunum to increase the synthesis of **Calbindin-D9k** (a calcium-binding protein), **TRPV6** (calcium channels), and **Ca-ATPase**. These proteins facilitate the active transport of calcium from the gut lumen into the bloodstream. **2. Why the other options are incorrect:** * **Parathyroid Hormone (PTH):** While PTH is the master regulator of calcium, it does **not** have a direct effect on the gut. Instead, it acts on the kidneys to stimulate the enzyme **1-alpha-hydroxylase**, which converts 25-hydroxyvitamin D into active 1,25-dihydroxycholecalciferol. Thus, PTH increases gut absorption *indirectly* via Vitamin D. * **Calcitonin:** Secreted by the parafollicular (C-cells) of the thyroid, calcitonin is a "hypocalcemic" hormone. It inhibits osteoclast activity and increases renal calcium excretion. It has no significant role in enhancing gut absorption. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The conversion of 25(OH)D to 1,25(OH)₂D in the kidney is the rate-limiting step in Vitamin D activation. * **Vitamin D Receptors (VDR):** These are nuclear receptors; therefore, Vitamin D acts by altering gene transcription. * **Other factors increasing absorption:** Acidic pH (Gastric acid), Lactose, and Amino acids (Arginine, Lysine). * **Factors decreasing absorption:** Oxalates, Phytates, Phosphates, and Glucocorticoids.

Q: Which of the following is NOT a hydrophilic hormone acting on cytosolic receptors?

Thyroxine. To answer this question correctly, one must understand the relationship between a hormone's chemical nature (solubility) and its receptor location. ### **Explanation of the Correct Answer** **A. Thyroxine (T4):** This is the correct answer because it is **lipophilic** (lipid-soluble), not hydrophilic. Although derived from the amino acid tyrosine, thyroid hormones are unique because they cross the cell membrane via transporters and bind to **nuclear receptors** (not cytosolic) to alter gene transcription. ### **Analysis of Incorrect Options** * **B. Epinephrine:** This is a catecholamine and is **hydrophilic**. It cannot cross the lipid bilayer and must act on **cell surface receptors** (G-protein coupled receptors). * **C. Growth Hormone (GH):** This is a large peptide hormone and is **hydrophilic**. It acts on **cell surface receptors** (specifically JAK-STAT linked receptors). * **D. Atrial Natriuretic Peptide (ANP):** This is a peptide hormone and is **hydrophilic**. It acts on **cell surface receptors** associated with particulate guanylyl cyclase. **Note on the Question Stem:** The question asks for a hormone that is *NOT* hydrophilic acting on *cytosolic* receptors. While options B, C, and D are hydrophilic, they act on **membrane** receptors. Thyroxine is the "most" correct answer because it is fundamentally hydrophobic/lipophilic. ### **High-Yield NEET-PG Pearls** 1. **Lipophilic Hormones (Intracellular Receptors):** * **Cytosolic Receptors:** Steroids (Glucocorticoids, Mineralocorticoids, Progesterone, Testosterone). *Mnemonic: "C" for Cortisol and Cytosol.* * **Nuclear Receptors:** Thyroid hormones (T3/T4), Estrogen, Vitamin D, and Retinoic Acid. 2. **Hydrophilic Hormones (Cell Surface Receptors):** * **cAMP pathway:** ACTH, Glucagon, TSH, PTH. * **IP3/DAG pathway:** Oxytocin, GnRH, TRH. * **Tyrosine Kinase pathway:** Insulin, IGF-1. * **JAK-STAT pathway:** GH, Prolactin, Erythropoietin.

Q: Which opioid is secreted from the anterior pituitary?

beta-endorphin. **Explanation:** The correct answer is **beta-endorphin**. The underlying medical concept revolves around the precursor molecule **Pro-opiomelanocortin (POMC)**. POMC is a large polypeptide synthesized in the corticotrophs of the anterior pituitary. Through the action of specific enzymes (prohormone convertases), POMC is cleaved into several biologically active peptides, including: 1. **ACTH** (Adrenocorticotropic Hormone) 2. **beta-lipotropin**, which is further cleaved into **beta-endorphin**. Beta-endorphin is an endogenous opioid peptide that binds primarily to mu-opioid receptors, playing a significant role in pain modulation and the "reward" system. **Analysis of Incorrect Options:** * **Growth Hormone (GH):** Secreted by somatotrophs in the anterior pituitary, but it is a peptide hormone, not an opioid. * **ACTH:** While ACTH is derived from the same precursor (POMC) as beta-endorphin, it is a trophic hormone that stimulates the adrenal cortex; it does not possess opioid activity. * **MSH (Melanocyte-Stimulating Hormone):** Also a derivative of POMC (specifically alpha-MSH is derived from the ACTH segment), but its primary function is skin pigmentation, not opioid signaling. **High-Yield Facts for NEET-PG:** * **POMC Derivatives:** Remember the "3 Ms" and "A" – MSH, Melancortin, beta-endorphin, and ACTH. * **Clinical Correlation:** In conditions like **Addison’s Disease**, high levels of ACTH (due to lack of feedback) are accompanied by high levels of MSH and beta-endorphin, leading to characteristic hyperpigmentation. * **Receptor Affinity:** Beta-endorphin has the highest affinity for **mu (μ) receptors** among the endogenous opioids.

Q: Gigantism is due to:

Increased secretion of growth hormone. ### Explanation **Correct Answer: C. Increased secretion of growth hormone** **Medical Concept:** Gigantism is a clinical condition caused by the **excessive secretion of Growth Hormone (GH)**, typically due to a somatotroph adenoma of the anterior pituitary. The defining feature of Gigantism is that this hypersecretion occurs **before the fusion of the epiphyseal plates** (pre-puberty). Because the long bones still have active growth plates, the excess GH (acting via IGF-1) stimulates dramatic linear bone growth, leading to tall stature and increased body proportions. **Analysis of Incorrect Options:** * **Options A & B (Cortisol):** Cortisol is a glucocorticoid from the adrenal cortex. Excess cortisol leads to **Cushing’s Syndrome** (characterized by weight gain and stunted growth in children), while a deficiency leads to **Addison’s Disease**. Neither causes gigantism. * **Option D (Decreased GH):** A deficiency of growth hormone during childhood leads to **Pituitary Dwarfism**, characterized by short stature and delayed skeletal maturation. **NEET-PG High-Yield Pearls:** * **Gigantism vs. Acromegaly:** Both are caused by GH excess. If it occurs *before* epiphyseal fusion, it is **Gigantism** (increased height). If it occurs *after* fusion, it is **Acromegaly** (thickening of bones, enlarged hands/feet, and soft tissue overgrowth, but no increase in height). * **Mediator:** GH does not act directly on bones for linear growth; it stimulates the liver to produce **IGF-1 (Somatomedin C)**, which is the actual mediator of bone growth. * **Screening Test:** The best initial screening test for GH excess is measuring **Serum IGF-1 levels** (due to its stable half-life). * **Confirmatory Test:** The gold standard is the **Oral Glucose Tolerance Test (OGTT)**; failure of GH to suppress below 1 ng/mL after glucose load confirms the diagnosis.

Q: Which steroid hormone is primarily involved in regulating water balance in the body?

Aldosterone. **Explanation:** **Aldosterone** is the correct answer because it is the primary mineralocorticoid secreted by the *zona glomerulosa* of the adrenal cortex. Its fundamental role is the maintenance of extracellular fluid volume and blood pressure. It acts on the **principal cells** of the distal convoluted tubule and collecting ducts to increase sodium reabsorption and potassium secretion. Since water follows sodium osmotically (obligatory water reabsorption), aldosterone effectively increases water retention, thereby regulating the body's water balance. **Analysis of Incorrect Options:** * **Estrogen:** While estrogens can cause some salt and water retention (often noticed during menstrual cycles), their primary role is the development of female secondary sexual characteristics and reproductive regulation. * **Testosterone:** This is an androgen responsible for male secondary sexual characteristics and spermatogenesis; it has negligible effects on acute water balance. * **Vitamin D (Calcitriol):** Though structurally a steroid hormone, its primary function is the regulation of **calcium and phosphate** homeostasis by increasing intestinal absorption, not water balance. **NEET-PG High-Yield Pearls:** * **Mechanism of Action:** Aldosterone binds to intracellular mineralocorticoid receptors (MR), leading to the up-regulation of **ENaC** (Epithelial Sodium Channels) and the **Na+/K+ ATPase** pump. * **Primary Stimulus:** The most potent stimulators of aldosterone secretion are **Hyperkalemia** (increased K+) and **Angiotensin II**. * **Conn’s Syndrome:** Primary hyperaldosteronism characterized by the triad of hypertension, hypokalemia, and metabolic alkalosis. * **Spironolactone:** A potassium-sparing diuretic that acts as a competitive antagonist to the aldosterone receptor.

Q: Hypothyroidism causes central nervous system features due to the presence of which receptor in the brain?

TR beta 2. **Explanation:** Thyroid hormones act through nuclear receptors known as **Thyroid Hormone Receptors (TR)**, which are encoded by two genes: *THRA* (Alpha) and *THRB* (Beta). **Why TR-beta 2 is the correct answer:** The **TR-beta 2 (TRβ2)** isoform is highly specific in its distribution. It is primarily expressed in the **Hypothalamus** and the **Anterior Pituitary gland**. It plays a critical role in the negative feedback mechanism, regulating the secretion of Thyrotropin-Releasing Hormone (TRH) and Thyroid-Stimulating Hormone (TSH). In hypothyroidism, the lack of T3/T4 binding to TRβ2 leads to the neurological and neuroendocrine manifestations associated with the central nervous system's feedback loop. **Analysis of Incorrect Options:** * **TR-alpha 1 (TRα1):** This is the most abundant receptor in the **Heart** and **Skeletal Muscle**. It is responsible for the chronotropic and inotropic effects on the heart. * **TR-alpha 2 (TRα2):** Although widely expressed in the brain, it is considered an **orphan receptor**. It does not bind T3 and actually acts as an antagonist to the effects of other thyroid receptors. * **TR-beta 1 (TRβ1):** This is the predominant isoform found in the **Liver** and **Kidneys**. It is primarily responsible for the metabolic effects of thyroid hormones, such as cholesterol lowering. **High-Yield Clinical Pearls for NEET-PG:** * **Resistance to Thyroid Hormone (RTH) Syndrome:** Usually caused by mutations in the **TR-beta** gene. Patients present with high T3/T4 levels but inappropriately normal or high TSH. * **Most abundant TR in the Brain:** While TRβ2 is specific to the feedback centers, **TRα1** is the most widely distributed isoform in the brain overall, but TRβ2 is the classic answer for CNS-specific endocrine regulation. * **Metabolic Rate:** TR-beta is the primary mediator of the thermogenic and metabolic effects in the liver.

Question 1

Which of the following is a lipophilic hormone that acts on nuclear receptors, while the others are hydrophilic hormones that act on cytosolic receptors?

Accepted Answer

Thyroxine

Answer

Epinephrine

Answer

Growth Hormone (GH)

Answer

Adrenocorticotropic Hormone (ACTH)

Question 2

Calcium absorption from the gut is enhanced by which of the following?

Accepted Answer

1,25-dihydroxycholecalciferol

Answer

Parathyroid hormone

Answer

Calcitonin

Answer

All of the above

Question 3

Which of the following is NOT a hydrophilic hormone acting on cytosolic receptors?

Accepted Answer

Thyroxine

Answer

Epinephrine

Answer

Growth Hormone (GH)

Answer

Atrial Natriuretic Peptide (ANP)

Question 4

Which opioid is secreted from the anterior pituitary?

Accepted Answer

beta-endorphin

Answer

Growth Hormone (GH)

Answer

Adrenocorticotropic Hormone (ACTH)

Answer

Melanocyte-Stimulating Hormone (MSH)

Question 5

Gigantism is due to:

Accepted Answer

Increased secretion of growth hormone

Answer

Increased secretion of cortisol

Answer

Decreased secretion of cortisol

Answer

Decreased secretion of growth hormone

Question 6

Decreased activity of type I 5’-monodeiodinase could lead to which physiologic effect?

Accepted Answer

Increased plasma reverse T3

Answer

Increased plasma triiodothyronine (T3)

Answer

Decreased plasma thyroxine T4

Answer

Increased TSH

Question 7

Which steroid hormone is primarily involved in regulating water balance in the body?

Accepted Answer

Aldosterone

Answer

Estrogen

Answer

Testosterone

Answer

Vitamin D

Question 8

A 35-year-old woman presents with heat intolerance and menstrual abnormalities. On examination, a diffusely enlarged thyroid gland is present. Which of the following features will not be present in her?

Accepted Answer

Decreased protein breakdown

Answer

Increased heart rate

Answer

Increased stroke volume

Answer

Decreased peripheral vascular resistance

Question 9

Hypothyroidism causes central nervous system features due to the presence of which receptor in the brain?

Accepted Answer

TR beta 2

Answer

TR alpha 1

Answer

TR alpha 2

Answer

TR beta 1

Question 10

Accidental transection of the pituitary stalk can cause which of the following?

Accepted Answer

Diabetes mellitus

Answer

Polyuria

Answer

Galactorrhea

Answer

Diabetes insipidus

Endocrinology — MCQs

Endocrinology — MCQs

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