Endocrinology Practice Questions

Q: Which of the following is the most useful investigation for thyroid function?

TSH. **Explanation:** **Serum TSH (Thyroid Stimulating Hormone)** is considered the single most useful and sensitive initial screening test for evaluating thyroid function in the general population. **Why TSH is the Correct Answer:** The relationship between Serum TSH and Free T4 is **log-linear**; even a minor change in the concentration of free thyroid hormones results in a much larger, compensatory change in TSH levels from the anterior pituitary. Therefore, TSH is the first parameter to become abnormal in both subclinical hypothyroidism (high TSH) and subclinical hyperthyroidism (low TSH), often before the patient becomes symptomatic or T3/T4 levels fall outside the reference range. **Why Other Options are Incorrect:** * **T3 (Triiodothyronine):** While T3 is the biologically active form, it is the least sensitive for screening. In early hypothyroidism, T3 levels are often maintained within the normal range due to increased TSH stimulation. It is primarily useful for diagnosing T3-toxicosis. * **T4 (Thyroxine):** Total T4 levels are heavily influenced by changes in **Thyroxine-Binding Globulin (TBG)** levels (e.g., pregnancy, OCP use). While Free T4 is more accurate, it is still less sensitive than TSH for early detection of thyroid dysfunction. * **TRH (Thyrotropin-Releasing Hormone):** TRH stimulation tests are rarely used in modern clinical practice. They were historically used to distinguish between secondary and tertiary hypothyroidism but have been largely replaced by high-sensitivity TSH assays and MRI. **High-Yield Clinical Pearls for NEET-PG:** * **Best Screening Test:** TSH. * **Best test to monitor Thyroid Replacement (Levothyroxine):** TSH (wait 6–8 weeks after dose adjustment). * **Exception:** In **Secondary (Central) Hypothyroidism**, TSH is unreliable; **Free T4** is the investigation of choice to monitor treatment. * **Best test for early Hyperthyroidism:** TSH (suppressed).

Q: Sclerostin is produced by which type of bone cell?

Osteocytes. **Explanation:** **Correct Answer: A. Osteocytes** Sclerostin is a glycoprotein primarily secreted by **mature osteocytes** (cells embedded within the mineralized bone matrix). It acts as a potent negative regulator of bone formation. Mechanistically, sclerostin binds to LRP5/6 receptors on the surface of osteoblasts, thereby inhibiting the **Wnt signaling pathway**. This inhibition prevents osteoblast proliferation and differentiation, leading to decreased bone formation. When bone experiences mechanical loading, sclerostin production decreases, allowing Wnt signaling to trigger bone deposition. **Why other options are incorrect:** * **B. Osteoblasts:** While osteoblasts are the targets of sclerostin action, they do not produce it. Osteoblasts are responsible for bone matrix synthesis (osteoid). * **C. Osteoclasts:** These are myeloid-derived cells responsible for bone resorption. They do not produce sclerostin; however, sclerostin indirectly promotes their activity by increasing the RANKL/Osteoprotegerin (OPG) ratio. * **D. Chondrocytes:** These are cells found in cartilage. While they share a mesenchymal origin with bone cells, they are not the source of sclerostin in the context of bone remodeling. **High-Yield Clinical Pearls for NEET-PG:** * **Romosozumab:** A monoclonal antibody against sclerostin used in the treatment of severe osteoporosis. It has a dual effect: increasing bone formation and decreasing bone resorption. * **Van Buchem Disease & Sclerosteosis:** Rare genetic conditions caused by a deficiency in sclerostin, leading to excessive bone overgrowth (hyperostosis). * **The "Master Regulator":** Osteocytes are now considered the "master regulators" of bone remodeling because they sense mechanical strain and coordinate both osteoblast and osteoclast activity via sclerostin and RANKL.

Q: Which hormone acts on intracellular receptors?

Thyroxine. **Explanation:** Hormone receptors are classified based on their location, which is determined by the hormone's chemical nature (solubility). **1. Why Thyroxine is Correct:** Thyroxine ($T_4$) and Triiodothyronine ($T_3$) are lipid-soluble hormones. Unlike most amino acid-derived hormones, thyroid hormones enter the cell via carrier-mediated transport and bind to **intracellular receptors** (specifically **nuclear receptors**). Once bound, they act as transcription factors to alter gene expression. Other hormones using intracellular receptors include Steroids (Cortisol, Aldosterone, Estrogen, Progesterone, Testosterone), Vitamin D, and Retinoic acid. **2. Why the Other Options are Incorrect:** * **Glucagon:** A peptide hormone that binds to **G-Protein Coupled Receptors (GPCR)** on the cell membrane, utilizing the cAMP second messenger system. * **Insulin:** A peptide hormone that binds to a transmembrane **Enzyme-linked receptor** (specifically **Receptor Tyrosine Kinase**). * **PDGF (Platelet-Derived Growth Factor):** Like insulin, this growth factor binds to membrane-bound **Receptor Tyrosine Kinase** to trigger cellular proliferation. **Clinical Pearls for NEET-PG:** * **Mnemonic for Intracellular Receptors:** **"PET TV"** – **P**rogesterone, **E**strogen, **T**estosterone, **T**hyroid hormones ($T_3/T_4$), and **V**itamin D/A. * **Location Nuance:** While Steroids (like Cortisol) often bind to **cytoplasmic** receptors before translocating to the nucleus, Thyroid hormones bind directly to receptors already located on the **chromatin in the nucleus**. * **Speed of Action:** Hormones acting on intracellular receptors have a **slow onset** (hours to days) because they require protein synthesis, whereas membrane-bound receptors (like Glucagon) trigger rapid enzymatic cascades.

Q: What happens to FSH and LH concentrations at birth?

They rise abruptly.. **Explanation:** The correct answer is **A. They rise abruptly.** **1. Why the correct answer is right:** During intrauterine life, the fetal hypothalamus-pituitary-gonadal (HPG) axis is functional but remains profoundly suppressed. This suppression is due to the **negative feedback** exerted by high levels of maternal and placental hormones (estrogen and progesterone) crossing the placenta. At birth, the sudden removal of the placenta leads to a sharp decline in these circulating maternal steroids. This "withdrawal" relieves the negative feedback on the infant's pituitary, causing an **abrupt rise** in the secretion of Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH). This phenomenon is often referred to as **"Mini-puberty."** **2. Why the incorrect options are wrong:** * **Option B:** FSH and LH do not fall; rather, it is the *maternal steroids* that fall, which triggers the rise in gonadotropins. * **Option C:** A "no change" status is incorrect because the hormonal environment transitions from a suppressed state to an active state immediately post-delivery. * **Option D:** While gonadotropin levels remain elevated for several months (peaking around 1–3 months), the initial surge occurs **at birth** due to the loss of placental inhibition. **3. NEET-PG High-Yield Pearls:** * **Mini-puberty:** In males, the LH surge stimulates Leydig cells, leading to a testosterone peak between 1–3 months of age (essential for genital development). In females, FSH remains elevated longer (up to 2–3 years of age). * **Juvenile Pause:** After the initial postnatal surge, gonadotropin levels fall and remain low until the onset of true puberty (due to high sensitivity of the gonadostat to low steroid levels). * **Clinical Correlation:** This postnatal surge is why some neonates may show transient breast engorgement or "witch’s milk" (though this is primarily due to prolactin and steroid withdrawal).

Q: Which hormone secretion is increased if the pituitary stalk is transected?

Prolactin. ### Explanation **1. Why Prolactin is the Correct Answer:** The secretion of most anterior pituitary hormones is primarily regulated by **releasing hormones** from the hypothalamus. However, **Prolactin (PRL)** is unique because its primary hypothalamic control is **tonic inhibition**. The hypothalamus secretes **Dopamine**, which travels through the hypothalamic-hypophyseal portal system to the anterior pituitary to inhibit prolactin release. When the **pituitary stalk is transected**, the delivery of dopamine is interrupted. This "removal of the inhibitory brake" leads to an autonomous, unregulated increase in prolactin secretion. This phenomenon is known as **Stalk Effect hyperprolactinemia**. **2. Why the Other Options are Incorrect:** * **B (ACTH) and C (GH):** These hormones depend on **stimulatory** signals from the hypothalamus (CRH for ACTH; GHRH for GH). If the stalk is transected, these stimulatory signals cannot reach the pituitary, leading to a **decrease** or cessation of their secretion. * **D (None):** This is incorrect because Prolactin levels consistently rise following stalk interruption. **3. High-Yield NEET-PG Clinical Pearls:** * **Dopamine = Prolactin Inhibiting Hormone (PIH):** It acts on **D2 receptors** located on lactotrophs. * **Posterior Pituitary Hormones:** Stalk transection also leads to **Diabetes Insipidus** because ADH and Oxytocin (synthesized in the hypothalamus) can no longer be transported to the posterior pituitary for storage and release. * **Clinical Correlation:** Any mass lesion (like a Craniopharyngioma) compressing the pituitary stalk can cause mild elevations in Prolactin (usually <100 ng/mL), mimicking a small prolactinoma. * **Rule of Thumb:** In pituitary stalk injury, **all** anterior pituitary hormones decrease **EXCEPT** Prolactin, which increases.

Q: Which of the following is NOT a function of oxytocin?

Galactopoiesis. **Explanation:** The correct answer is **D. Galactopoiesis**. To understand this, one must distinguish between the different stages of lactation. **Galactopoiesis** refers to the maintenance of milk secretion once it has been established. This process is primarily regulated by **Prolactin** (from the anterior pituitary) and growth hormone, not oxytocin. **Analysis of Options:** * **A. Galactokinesis:** This refers to the "milk-ejection reflex" or "let-down reflex." Oxytocin causes the contraction of **myoepithelial cells** surrounding the mammary alveoli, squeezing milk into the ducts. This is a classic function of oxytocin. * **B. Uterine contraction in a non-pregnant female:** While oxytocin is famous for labor, it also causes rhythmic uterine contractions during menstruation and coitus (to aid sperm transport) in non-pregnant females. * **C. Contraction of smooth muscle of vas deferens:** In males, oxytocin is released during ejaculation and facilitates sperm transport by stimulating the contraction of the smooth muscles in the vas deferens and epididymis. **High-Yield NEET-PG Pearls:** 1. **Synthesis vs. Storage:** Oxytocin is synthesized in the **Paraventricular nucleus (PVN)** of the hypothalamus but stored and released by the **Posterior Pituitary (Neurohypophysis)**. 2. **The "Love Hormone":** It plays a crucial role in maternal-infant bonding and social recognition. 3. **Positive Feedback:** The Ferguson reflex (vaginal/cervical stretch) and suckling are the primary triggers for oxytocin release, representing rare examples of positive feedback in physiology. 4. **Receptor Type:** Oxytocin acts via **G-protein coupled receptors (Gq)**, leading to increased intracellular calcium.

Q: All of the following steps in the synthesis of thyroid hormones are catalyzed by the thyroid peroxidase enzyme except?

Iodide trapping. **Explanation:** The synthesis of thyroid hormones is a multi-step process, most of which occurs within the follicular lumen and is mediated by the membrane-bound enzyme **Thyroid Peroxidase (TPO)**. **Why Iodide Trapping is the correct answer:** Iodide trapping is the first step in thyroid hormone synthesis, where inorganic iodide ($I^-$) is transported from the blood into the follicular cell against a chemical and electrical gradient. This process is mediated by the **Sodium-Iodide Symporter (NIS)**, a secondary active transporter located on the basolateral membrane. It is **not** catalyzed by TPO. **Analysis of incorrect options (Steps catalyzed by TPO):** * **Oxidation:** TPO uses hydrogen peroxide ($H_2O_2$) to oxidize iodide ($I^-$) into iodine ($I^0$ or $I^+$), the reactive form required for binding. * **Organification (Iodination):** TPO catalyzes the binding of oxidized iodine to tyrosine residues on the thyroglobulin molecule to form Monoiodotyrosine (MIT) and Diiodotyrosine (DIT). * **Coupling:** TPO facilitates the phenolic coupling of iodotyrosines. Two DIT molecules form $T_4$, while one MIT and one DIT form $T_3$. **High-Yield NEET-PG Pearls:** * **Wolff-Chaikoff Effect:** An autoregulatory phenomenon where high levels of circulating iodide inhibit TPO, leading to a transient decrease in thyroid hormone synthesis. * **Pendrin:** A chloride-iodide exchanger on the apical membrane that transports iodide into the follicle lumen. Mutations lead to **Pendred Syndrome** (goiter and sensorineural deafness). * **Clinical Correlation:** Propylthiouracil (PTU) and Methimazole act by inhibiting the TPO enzyme. PTU additionally inhibits peripheral conversion of $T_4$ to $T_3$.

Q: Glucose is transported in the pancreas through which receptor?

GLUT 2. **Explanation:** The correct answer is **GLUT 2**. In the pancreatic beta cells, GLUT 2 acts as the primary glucose sensor. It is a high-capacity, low-affinity bidirectional transporter. Because of its high $K_m$ (low affinity), the rate of glucose entry into the beta cell is proportional to the blood glucose concentration within the physiological range. Once inside, glucose is phosphorylated by **Glucokinase**, leading to ATP production, closure of ATP-sensitive $K^+$ channels, depolarization, and subsequent insulin release. **Analysis of Incorrect Options:** * **GLUT 1:** Found primarily in **RBCs** and the **Blood-Brain Barrier**. It provides basal glucose uptake required to sustain respiration in all cells. * **GLUT 3:** Primarily located in **Neurons** and the placenta. It has a very low $K_m$ (high affinity), ensuring glucose uptake even during hypoglycemia. * **GLUT 4:** The only **Insulin-dependent** glucose transporter. it is found in **Skeletal muscle** and **Adipose tissue**. Insulin triggers the translocation of GLUT 4 from intracellular vesicles to the plasma membrane. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT 2 Locations:** Remember the mnemonic **"KILP"** — **K**idney (PCT), **I**ntestine (basolateral membrane), **L**iver, and **P**ancreas. * **SGLT vs. GLUT:** SGLT (Sodium-Glucose Linked Transporters) are involved in *active* transport (secondary active), whereas GLUTs are involved in *facilitated diffusion*. * **Fanconi-Bickel Syndrome:** A rare glycogen storage disease caused by a congenital defect in the **GLUT 2** transporter. * **Glucokinase:** Often called the "Glucose Sensor" of the pancreas; mutations here can lead to MODY (Maturity-Onset Diabetes of the Young).

Question 1

Which of the following hormones exerts the least effect on the calcium metabolism of bone tissue?

Accepted Answer

Norepinephrine

Answer

Androgen

Answer

Estrogen

Answer

Thyroid hormone

Question 2

An excess of which of the following hormones may be associated with increased sensitivity to epinephrine?

Accepted Answer

Thyroid hormone

Answer

Testosterone

Answer

Parathyroid hormone

Answer

Insulin

Question 3

Which of the following is the most useful investigation for thyroid function?

Accepted Answer

TSH

Answer

T3

Answer

T4

Answer

TRH

Question 4

Sclerostin is produced by which type of bone cell?

Accepted Answer

Osteocytes

Answer

Osteoblasts

Answer

Osteoclasts

Answer

Chondrocytes

Question 5

Which hormone acts on intracellular receptors?

Accepted Answer

Thyroxine

Answer

Glucagon

Answer

Insulin

Answer

PDGF

Question 6

What happens to FSH and LH concentrations at birth?

Accepted Answer

They rise abruptly.

Answer

They fall abruptly.

Answer

There is no change.

Answer

They rise after 3 months of age.

Question 7

Which hormone secretion is increased if the pituitary stalk is transected?

Accepted Answer

Prolactin

Answer

ACTH

Answer

GH

Answer

None

Question 8

Which of the following is NOT a function of oxytocin?

Accepted Answer

Galactopoiesis

Answer

Galactokinesis

Answer

Uterine contraction in a non-pregnant female

Answer

Contraction of smooth muscle of vas deferens

Question 9

All of the following steps in the synthesis of thyroid hormones are catalyzed by the thyroid peroxidase enzyme except?

Accepted Answer

Iodide trapping

Answer

Coupling

Answer

Oxidation

Answer

Organification

Question 10

Glucose is transported in the pancreas through which receptor?

Accepted Answer

GLUT 2

Answer

GLUT 1

Answer

GLUT 3

Answer

GLUT 4

Endocrinology — MCQs

Endocrinology — MCQs

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