Endocrinology Practice Questions

Q: Which of the following inhibits prolactin secretion?

Bromocriptine. **Explanation:** The regulation of prolactin is unique among anterior pituitary hormones because it is under **predominant tonic inhibition** by the hypothalamus. The primary Prolactin Inhibiting Factor (PIF) is **Dopamine**, which acts on D2 receptors of the lactotrophs. **Why Bromocriptine is correct:** **Bromocriptine** is a potent **Dopamine (D2) receptor agonist**. By mimicking the action of endogenous dopamine, it directly inhibits the synthesis and secretion of prolactin from the anterior pituitary. It is the clinical gold standard for treating hyperprolactinemia and prolactinomas. **Analysis of Incorrect Options:** * **A. Dopamine antagonist:** These drugs (e.g., Metoclopramide, Haloperidol) block the inhibitory effect of dopamine, leading to **increased** prolactin levels (hyperprolactinemia) and potentially causing galactorrhea. * **B. GABA:** While GABA can have minor inhibitory effects on prolactin in some experimental models, it is not the primary physiological regulator. In the context of standard physiology and NEET-PG, Dopamine is the definitive inhibitor. * **C. Neurophysin:** These are carrier proteins for Oxytocin (Neurophysin I) and ADH (Neurophysin II). They are involved in the transport of posterior pituitary hormones and have no role in prolactin regulation. **High-Yield Clinical Pearls for NEET-PG:** * **TRH (Thyrotropin-Releasing Hormone):** Acts as a potent prolactin-releasing factor. This explains why patients with **primary hypothyroidism** (high TRH) often present with secondary hyperprolactinemia. * **Prolactin & GnRH:** High prolactin levels inhibit the pulsatile release of GnRH, leading to infertility and amenorrhea. * **Suckling Reflex:** Inhibits dopamine release, thereby disinhibiting prolactin to promote milk production.

Q: Which of the following is NOT a mechanism of action of insulin?

Inhibition of Na+K+ATPase. **Explanation:** The correct answer is **C (Inhibition of Na+K+ATPase)** because insulin actually **stimulates** the Na+K+ATPase pump, rather than inhibiting it. **Why Option C is correct:** Insulin increases the activity and number of Na+K+ATPase pumps on cell membranes (especially in skeletal muscle and adipose tissue). This leads to an increased influx of Potassium ($K^+$) into the cells and an efflux of Sodium ($Na^+$). This mechanism is clinically significant as it can lead to hypokalemia, which is why insulin is used in the emergency management of hyperkalemia. **Why the other options are incorrect:** * **Option A (Tyrosine kinase-beta cell stimulation):** The insulin receptor is a heterotetramer with intrinsic **Tyrosine Kinase** activity. When insulin binds to the alpha subunits, it causes autophosphorylation of the beta subunits, triggering the intracellular signaling cascade. * **Option B (Incorporation of GLUT-4):** Insulin promotes the translocation of **GLUT-4** (glucose transporter) from intracellular vesicles to the plasma membrane in skeletal muscle and adipose tissue, facilitating glucose uptake. * **Option D (Hexokinase stimulation):** Insulin enhances glucose utilization by inducing and activating key glycolytic enzymes, including **Hexokinase** (and Glucokinase in the liver), to trap glucose inside the cell as Glucose-6-Phosphate. **High-Yield NEET-PG Pearls:** * **GLUT-4** is the only insulin-dependent glucose transporter. * Insulin causes a **"Shift" of Potassium** into cells; always monitor $K^+$ levels when administering an insulin drip for DKA. * **Anabolic effects:** Insulin stimulates protein synthesis, glycogen synthesis (Glycogen Synthase), and lipogenesis, while inhibiting gluconeogenesis and ketogenesis.

Q: Which of the following is an amine hormone?

Thyroxine. **Explanation:** Hormones are chemically classified into three main categories: **Amines, Peptides/Proteins, and Steroids.** **Thyroxine (T4)** is the correct answer because it is an **amine hormone**. Amine hormones are derivatives of the amino acid **Tyrosine**. This group includes thyroid hormones (T3, T4) and catecholamines (Epinephrine, Norepinephrine, and Dopamine). Although derived from tyrosine, thyroid hormones are unique because they are lipophilic and act on intracellular receptors, unlike catecholamines which are hydrophilic. **Analysis of Incorrect Options:** * **Insulin:** This is a **peptide hormone** consisting of 51 amino acids arranged in two chains (A and B) linked by disulfide bridges. * **Glucocorticoids (e.g., Cortisol):** These are **steroid hormones** derived from cholesterol. They are produced in the adrenal cortex and are lipid-soluble. * **Parathyroid Hormone (PTH):** This is a **polypeptide hormone** (84 amino acids) secreted by the chief cells of the parathyroid gland. **High-Yield NEET-PG Pearls:** 1. **Tyrosine Derivatives:** Remember the "T" rule—Tyrosine gives rise to **T**hyroid hormones and **T**ransmitters (Catecholamines). 2. **Tryptophan Derivative:** **Melatonin** and Serotonin are derived from the amino acid Tryptophan (frequently tested). 3. **Receptor Location:** Most amine and peptide hormones bind to cell surface receptors, but **Thyroid hormones** are the notable exception—they bind to **nuclear receptors**. 4. **Steroid Mnemonic:** All hormones from the Adrenal Cortex, Gonads, and Corpus Luteum are steroids (plus Vitamin D).

Q: A patient has a goiter associated with high plasma levels of both TRH and TSH. Her heart rate is elevated. This patient most likely has which condition?

A hypothalamic tumor secreting large amounts of TRH. ### Explanation **Concept Overview:** The diagnosis of thyroid disorders relies on understanding the **Hypothalamic-Pituitary-Thyroid (HPT) axis** and the principle of **negative feedback**. Normally, high levels of thyroid hormones (T3/T4) inhibit the release of TRH from the hypothalamus and TSH from the anterior pituitary. **Why Option B is Correct:** In this patient, both **TRH and TSH are elevated**. This indicates a "top-down" pathology. A **hypothalamic tumor** secreting TRH will overstimulate the pituitary to produce TSH. High TSH, in turn, overstimulates the thyroid gland, causing **goiter** (hyperplasia) and excessive T3/T4 production. The elevated heart rate (tachycardia) is a clinical manifestation of the resulting hyperthyroidism. **Why Other Options are Incorrect:** * **A. Endemic Goiter:** Caused by iodine deficiency. While TSH is high (due to lack of feedback inhibition), T3/T4 levels are low, and TRH is usually not the primary driver measured in this context. * **C. Pituitary Tumor (TSH-oma):** While this would cause high TSH and high T3/T4, the high circulating thyroid hormones would **suppress** the hypothalamus via negative feedback, leading to **low TRH** levels. * **D. Graves' Disease:** This is an autoimmune condition where TSH-receptor antibodies (TRAb) stimulate the thyroid. Because T3/T4 levels are very high, both **TRH and TSH would be suppressed (low)**. **NEET-PG High-Yield Pearls:** * **Secondary Hyperthyroidism:** High T3/T4 with high TSH (usually a pituitary adenoma). * **Tertiary Hyperthyroidism:** High T3/T4 with high TSH and high TRH (hypothalamic origin). * **Primary Hyperthyroidism:** High T3/T4 with low TSH (e.g., Graves', Toxic Multinodular Goiter). * **Wolff-Chaikoff Effect:** Reduction in thyroid hormone levels caused by ingestion of a large amount of iodine.

Q: Which of the following statements regarding thyroid hormones is correct?

Majority of the circulating T3 remains in bound form. ### Explanation **Correct Option: A. Majority of the circulating T3 remains in bound form** Thyroid hormones (T3 and T4) are highly hydrophobic and require carrier proteins for transport in the blood [1]. Approximately **99.7% of T3** and **99.97% of T4** circulate in bound form. The primary binding proteins are **Thyroxine-Binding Globulin (TBG)**, transthyretin, and albumin [1]. Only the unbound (free) fraction is biologically active and capable of entering target cells [1]. **Why the other options are incorrect:** * **Option B:** In reality, only about **20%** of circulating T3 is directly secreted by the thyroid gland. The remaining **80%** is produced via the peripheral deiodination of T4 (pro-hormone) by Type 1 and Type 2 deiodinase enzymes in tissues like the liver and kidney. * **Option C:** TSH is the **most sensitive** screening and diagnostic test for primary hypothyroidism. Due to the negative feedback loop, even a slight decrease in T4 levels leads to a logarithmic increase in TSH, often making it elevated even before T4 falls below the normal range (Subclinical Hypothyroidism). * **Option D:** The fetal pituitary-thyroid axis is **independent** of the maternal axis. The placenta is largely impermeable to TSH. While small amounts of maternal T4 cross the placenta to support early brain development, the fetus begins synthesizing its own thyroid hormones by the end of the first trimester (approx. 12 weeks). **High-Yield Clinical Pearls for NEET-PG:** * **Potency:** T3 is 3–4 times more potent than T4 and has a much shorter half-life (1 day vs. 7 days for T4) [2]. * **Wolff-Chaikoff Effect:** Transient inhibition of thyroid hormone synthesis due to high iodine intake. * **Amiodarone:** A high-yield drug that can cause both hypo- and hyperthyroidism due to its high iodine content and structural similarity to T4.

Q: Steroid hormones are believed to enter target cells via:

Simple diffusion. **Explanation:** **1. Why Simple Diffusion is Correct:** The plasma membrane is a phospholipid bilayer, which acts as a barrier to water-soluble substances but allows lipid-soluble molecules to pass through easily. **Steroid hormones** (such as cortisol, aldosterone, estrogen, and testosterone) are derivatives of **cholesterol**, making them highly **lipophilic (hydrophobic)**. Because of this lipid solubility, they can dissolve directly into the lipid bilayer and cross the cell membrane via **simple diffusion** along their concentration gradient. Once inside, they typically bind to intracellular receptors (cytoplasmic or nuclear) to initiate genomic effects. **2. Why the Other Options are Incorrect:** * **Facilitated Diffusion:** This requires specific transmembrane proteins (channels or carriers) to transport molecules that are too large or polar to cross the bilayer alone (e.g., glucose via GLUT). Steroids do not require these transporters. * **Carrier-mediated Endocytosis:** This is an active process involving the engulfing of substances (like LDL or Iron/Transferrin). While some steroid-binding proteins are endocytosed, the free hormone itself enters via diffusion. * **Cholesterol-lined Pores:** This is a distractor. There are no specific "cholesterol-lined" anatomical pores designed for hormone entry; the entire membrane is inherently permeable to them. **3. NEET-PG High-Yield Pearls:** * **Thyroid Hormones (T3, T4):** Despite being lipophilic, they primarily enter cells via **carrier-mediated transport** (MCT8/MCT10), not simple diffusion. This is a common "trap" question. * **Mechanism of Action:** Steroid hormones have a **slow onset** but **long duration** of action because they alter gene transcription (Mobile Receptor Hypothesis). * **Exception:** Some steroids can have rapid, non-genomic effects via membrane-bound receptors (e.g., certain effects of progesterone).

Q: Where are the alpha-estrogen receptors located?

Nuclear receptors in breast. **Explanation:** Estrogen receptors (ERs) are members of the **nuclear receptor superfamily** of intracellular transcription factors. Estrogen, being a lipid-soluble steroid hormone, diffuses across the cell membrane to bind with these receptors. Upon binding, the receptor-ligand complex translocates to the nucleus (if not already present), binds to Estrogen Response Elements (EREs) on DNA, and regulates gene transcription. **Why Option B is Correct:** There are two main subtypes of estrogen receptors: **ER-alpha (ERα)** and **ER-beta (ERβ)**. ERα is the predominant subtype found in the **breast** (specifically mammary epithelium) and the uterus. Its activation is responsible for the proliferative effects of estrogen in these tissues. **Analysis of Incorrect Options:** * **Options A & C:** While a small fraction of estrogen receptors (GPER) are membrane-bound for rapid non-genomic signaling, the classic alpha-estrogen receptors are fundamentally classified as **nuclear receptors**. * **Option D:** While ERα is present in the ovaries (theca cells), **ER-beta (ERβ)** is the predominant isoform found in the granulosa cells of the ovaries. **High-Yield NEET-PG Pearls:** * **Location Summary:** ERα is primarily in the breast, uterus, and liver; ERβ is primarily in the ovaries, prostate, and colon. * **Clinical Correlation:** ERα status is a critical biomarker in breast cancer; ER-positive tumors are candidates for endocrine therapies like **Tamoxifen** (a SERM). * **Mechanism:** Steroid receptors (Estrogen, Progesterone, Testosterone) are typically **Type I nuclear receptors**, whereas Thyroid and Vitamin D receptors are **Type II** (already bound to DNA).

Q: In hypogonadotropic hypogonadism, what is the typical hormonal profile?

Both LH and FSH are decreased. ### Explanation **Underlying Concept:** The Hypothalamic-Pituitary-Gonadal (HPG) axis operates through a feedback loop. In **Hypogonadotropic Hypogonadism**, the primary defect lies in the **Hypothalamus** (decreased GnRH) or the **Anterior Pituitary** (decreased LH/FSH). Because the "tropic" hormones (LH and FSH) that stimulate the gonads are low, the gonads fail to produce sex steroids (testosterone or estrogen). This is a **secondary** or **central** failure. **Why Option A is Correct:** In this condition, the pituitary fails to secrete adequate Gonadotropins. Therefore, both **Luteinizing Hormone (LH)** and **Follicle-Stimulating Hormone (FSH)** are **decreased** (or inappropriately normal) in the presence of low sex steroids. **Why Other Options are Incorrect:** * **Option B:** Increased LH and FSH with low sex steroids characterize **Hypergonadotropic Hypogonadism** (Primary failure, e.g., Klinefelter syndrome or Turner syndrome). Here, the lack of negative feedback from the gonads causes the pituitary to overproduce gonadotropins. * **Options C & D:** LH and FSH usually move in the same direction in central or primary failure. Isolated elevations or depressions are rare and typically point to specific conditions like PCOS (high LH:FSH ratio) or specific pituitary adenomas, rather than classic hypogonadotropic hypogonadism. **High-Yield Clinical Pearls for NEET-PG:** * **Kallmann Syndrome:** The most common congenital cause of hypogonadotropic hypogonadism, characterized by **anosmia** (failure of GnRH neurons to migrate from the olfactory bulb). * **Differential:** If LH/FSH are high, the problem is in the **Gonads** (Primary). If LH/FSH are low, the problem is in the **Pituitary/Hypothalamus** (Secondary). * **Prostate Cancer Treatment:** GnRH *agonists* (like Leuprolide) initially increase LH/FSH but eventually cause down-regulation of receptors, leading to a state of "chemical" hypogonadotropic hypogonadism.

Q: Exposure to darkness leads to increased melatonin secretion. What mechanism brings this about?

Increasing the serotonin N-acetyl transferase activity. **Explanation:** The pineal gland acts as a "neuroendocrine transducer," converting light signals from the retina into the hormonal signal of melatonin. **1. Why the correct answer is right:** Melatonin is synthesized from **Tryptophan** via the following pathway: *Tryptophan → 5-Hydroxytryptophan → Serotonin → N-Acetylserotonin → Melatonin.* The rate-limiting enzyme in this pathway is **Serotonin N-acetyltransferase (SNAT)**. In darkness, postganglionic sympathetic fibers release **Norepinephrine**, which acts on **β-adrenergic receptors** in the pineal gland. This increases intracellular cAMP, which significantly activates SNAT. Consequently, serotonin is rapidly converted to N-acetylserotonin, leading to a surge in melatonin production. **2. Why the incorrect options are wrong:** * **Option A:** The Suprachiasmatic Nucleus (SCN) is the "master clock." While light inhibits the SCN's stimulation of the pineal gland, the specific biochemical mechanism for increased melatonin is the *activation* of the enzymatic pathway, not merely a decrease in SCN activity. * **Option C:** Hydroxy-indole-O-methyltransferase (HIOMT) is the final enzyme in the pathway. While it is necessary, it is not the primary regulatory/rate-limiting step triggered by darkness. * **Option D:** Darkness **increases** (rather than blocks) the release of norepinephrine from the superior cervical ganglion to stimulate melatonin synthesis. **High-Yield Facts for NEET-PG:** * **Precursor:** L-Tryptophan. * **Rate-limiting enzyme:** Serotonin N-acetyltransferase (SNAT). * **Light pathway:** Retina → Retinohypothalamic tract → SCN → Intermediolateral column (T1-T2) → Superior Cervical Ganglion → Pineal Gland. * **Clinical Pearl:** Melatonin levels peak between **2 AM and 4 AM**. It is used clinically for jet lag and delayed sleep phase syndrome.

Question 1

Which of the following inhibits prolactin secretion?

Accepted Answer

Bromocriptine

Answer

Dopamine antagonist

Answer

GABA

Answer

Neurophysin

Question 2

Which of the following is NOT a mechanism of action of insulin?

Accepted Answer

Inhibition of Na+K+ATPase

Answer

Tyrosine kinase-beta cell stimulation

Answer

Incoporation of GLUT-4 into cells

Answer

Hexokinase stimulation

Question 3

Which of the following is an amine hormone?

Accepted Answer

Thyroxine

Answer

Insulin

Answer

Glucocorticoid

Answer

Parathyroid hormone (PTH)

Question 4

A patient has a goiter associated with high plasma levels of both TRH and TSH. Her heart rate is elevated. This patient most likely has which condition?

Accepted Answer

A hypothalamic tumor secreting large amounts of TRH

Answer

An endemic goiter

Answer

A pituitary tumor secreting large amounts of TSH

Answer

Graves' disease

Question 5

Parasympathetic stimulation of nerves innervating the islets of the pancreas will:

Accepted Answer

Increase secretion of insulin from B cells

Answer

Have no effect

Answer

Decrease secretion of insulin from B cells

Answer

Increase secretion of glucagon from B cells

Question 6

Which of the following statements regarding thyroid hormones is correct?

Accepted Answer

Majority of the circulating T3 remains in bound form

Answer

Only 50% of the circulating T3 is secreted by the thyroid

Answer

TSH estimation is not a sensitive test for the diagnosis of primary hypothyroidism

Answer

Fetal pituitary-thyroid axis is dependent to a large extent on the maternal pituitary-thyroid axis

Question 7

Steroid hormones are believed to enter target cells via:

Accepted Answer

Simple diffusion

Answer

Facilitated diffusion

Answer

Carrier-mediated endocytosis

Answer

Cholesterol-lined pores in the plasma membrane

Question 8

Where are the alpha-estrogen receptors located?

Accepted Answer

Nuclear receptors in breast

Answer

Membrane receptors in breast

Answer

Membrane receptors in ovaries

Answer

Nuclear receptors in ovaries

Question 9

In hypogonadotropic hypogonadism, what is the typical hormonal profile?

Accepted Answer

Both LH and FSH are decreased

Answer

Both LH and FSH are increased

Answer

LH is increased and FSH is decreased

Answer

LH is decreased and FSH is increased

Question 10

Exposure to darkness leads to increased melatonin secretion. What mechanism brings this about?

Accepted Answer

Increasing the serotonin N-acetyl transferase activity

Answer

Decreasing the activity of suprachiasmatic nuclei

Answer

Decreasing the hydroxy-indole-o-methyltransferase activity

Answer

Blocking the release of norepinephrine from sympathetic nerve terminals

Endocrinology — MCQs

Endocrinology — MCQs

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