Endocrinology Practice Questions

Q: What is the storage form of thyroid hormone?

Thyroglobulin. The thyroid gland is unique among endocrine glands because it stores its hormones extracellularly in the follicular lumen [1]. The correct answer is **Thyroglobulin (Tg)**, a large glycoprotein synthesized by follicular cells [1]. 1. **Why Thyroglobulin is correct:** After synthesis, Tg is secreted into the colloid. Within this colloid, iodine is attached to tyrosine residues on the Tg backbone (organification) to form MIT and DIT, which then couple to form T3 and T4 [2]. These hormones remain covalently bound to the thyroglobulin molecule, acting as a reservoir that can provide a 2–3 month supply of thyroid hormone [1]. 2. **Why other options are incorrect:** * **Tri-iodothyronine (T3):** This is the metabolically active form of the hormone, not the storage form. It is released into the blood after the proteolysis of thyroglobulin [1]. * **Tri-iodotyrosine:** This is a distractor term; it is not a standard intermediate in thyroid physiology. * **Di-iodotyrosine (DIT):** This is an intermediate precursor formed during the organification of iodine. While it is stored as part of the thyroglobulin complex, it is not the "storage form" itself but rather a building block [2]. **High-Yield Clinical Pearls for NEET-PG:** * **Wolff-Chaikoff Effect:** A transient decrease in thyroid hormone synthesis due to the ingestion of a large amount of iodine. * **Pendred Syndrome:** Characterized by sensorineural hearing loss and goiter due to a defect in the Pendrin iodine transporter. * *Thyroglobulin Levels:** Used clinically as a **tumor marker** to monitor for recurrence in patients with differentiated thyroid cancer (Papillary or Follicular) after total thyroidectomy.

Q: Which of the following iodinated compounds is present in the maximum concentration in the thyroid?

Diiodotyrosine (DIT). **Explanation:** The synthesis of thyroid hormones occurs within the thyroid follicle through the iodination of tyrosine residues on the protein **thyroglobulin**. This process, known as organification, involves the enzyme thyroid peroxidase (TPO). **Why DIT is the correct answer:** During the synthesis process, iodine is added to tyrosine to form Monoiodotyrosine (MIT) and Diiodotyrosine (DIT). Statistically and biochemically, the coupling process favors the formation of DIT. In the thyroid gland, the ratio of DIT to MIT is approximately **2:1**. Since T3 and T4 are stored in the colloid as part of the thyroglobulin complex, and DIT is the precursor for both T4 (DIT + DIT) and T3 (MIT + DIT), **Diiodotyrosine (DIT)** remains the most abundant iodinated compound stored within the gland. **Analysis of Incorrect Options:** * **A. Monoiodotyrosine (MIT):** While present in significant amounts, its concentration is roughly half that of DIT. * **C. Triiodothyronine (T3):** T3 is the most biologically active form, but it is produced in much smaller quantities within the gland compared to its precursors. Most circulating T3 is actually derived from the peripheral conversion of T4. * **D. Reverse T3:** This is a metabolically inactive product formed primarily in peripheral tissues (not the thyroid) during states of illness or starvation. **High-Yield Clinical Pearls for NEET-PG:** * **Wolff-Chaikoff Effect:** A transient reduction in thyroid hormone synthesis caused by the ingestion of a large amount of iodine. * **Storage:** The thyroid is unique among endocrine glands because it stores its hormones extracellularly (in the colloid) in quantities sufficient to last 2–3 months. * **Coupling:** 2 DIT = T4 (Thyroxine); 1 MIT + 1 DIT = T3. Since T4 is produced in a 10:1 ratio to T3, the requirement for DIT is significantly higher.

Q: Which of the following hormones is released from the hypothalamus?

Cortisol releasing hormone. The hypothalamus acts as the primary neuroendocrine control center, secreting "releasing" and "inhibiting" hormones that regulate the anterior pituitary gland via the hypophyseal portal system. ### **Explanation of the Correct Answer** **B. Cortisol Releasing Hormone (CRH):** This is the correct answer. CRH (also known as Corticotropin-Releasing Hormone) is synthesized by the parvocellular neurosecretory cells of the **paraventricular nucleus (PVN)** of the hypothalamus. It stimulates the anterior pituitary to release Adrenocorticotropic Hormone (ACTH), which in turn stimulates the adrenal cortex to produce cortisol. ### **Analysis of Incorrect Options** * **A. Orexin (Hypocretin):** While produced in the lateral hypothalamus, it functions primarily as a **neurotransmitter** regulating wakefulness and appetite, rather than a classical endocrine hormone released into the portal circulation. * **C. Neuropeptide Y (NPY):** This is a potent orexigenic (appetite-stimulating) peptide. While found in the arcuate nucleus of the hypothalamus, it is classified as a **neuromodulator/neurotransmitter** involved in energy homeostasis. * **D. Ghrelin:** Known as the "hunger hormone," it is primarily secreted by the **P/D1 cells of the stomach fundus**. It acts on the hypothalamus to increase appetite but is not produced there. ### **High-Yield Clinical Pearls for NEET-PG** * **The "Releasing" Rule:** Almost all hormones with "Releasing" in their name (GHRH, TRH, GnRH, CRH) originate from the hypothalamus. * **Posterior Pituitary Connection:** ADH (Vasopressin) and Oxytocin are *synthesized* in the hypothalamus (Supraoptic and Paraventricular nuclei) but *stored and released* by the posterior pituitary. * **Dopamine:** It is the primary **Prolactin-Inhibiting Hormone (PIH)** secreted by the hypothalamus. A deficiency in hypothalamic dopamine leads to hyperprolactinemia.

Q: Which of the following is an orexigenic hormone?

Ghrelin. ***Ghrelin (Correct Answer)*** - Ghrelin is often termed the "hunger hormone" as it acts primarily as a powerful **orexigenic signal**, stimulating appetite and food intake. - It is primarily produced by specialized cells in the **stomach**, and its levels typically rise before meals. *Melanocyte-stimulating hormone (Incorrect)* - MSH, specifically **alpha-MSH**, is released from **proopiomelanocortin (POMC) neurons** in the hypothalamus. - It acts to suppress appetite and is classified as an **anorexigenic hormone**. *Cholecystokinin (Incorrect)* - CCK is a **satiety hormone** released in response to fat and protein entering the duodenum and jejunum. - It acts on the brainstem and peripheral nerves to inhibit gastric emptying and induce short-term feelings of **satiety** (anorexigenic effect). *Leptin (Incorrect)* - Leptin is produced mainly by **adipocytes (fat cells)** and signals the brain about long-term energy stores. - High circulating levels of leptin act on the hypothalamus to **inhibit appetite** and increase energy expenditure, making it an **anorexigenic hormone (satiety signal)**.

Q: Which of the following tissues will not be able to take up glucose in insulin resistance/insulin absence/diabetes mellitus?

Skeletal muscle. ### **Explanation** The uptake of glucose into cells is mediated by **Glucose Transporters (GLUT)**. The fundamental concept here is the distinction between **Insulin-Dependent** and **Insulin-Independent** glucose uptake. **1. Why Skeletal Muscle is Correct:** * **Skeletal muscle** (and adipose tissue) primarily utilizes **GLUT-4**, which is the only **insulin-dependent** glucose transporter. * In the absence of insulin or in states of insulin resistance, GLUT-4 remains sequestered in intracellular vesicles and cannot translocate to the cell membrane. Consequently, these tissues cannot take up glucose effectively, leading to post-prandial hyperglycemia. **2. Why the Other Options are Incorrect:** * **Kidney (Option A):** Uses **GLUT-2** (basolateral membrane) and **SGLT-1/2** (apical membrane) for glucose reabsorption. These are insulin-independent. * **Brain (Option C):** Relies on **GLUT-1** and **GLUT-3**. The brain requires a continuous supply of glucose regardless of insulin levels to maintain metabolic function. * **Red Blood Cells (Option D):** Utilize **GLUT-1** for constitutive glucose uptake, which does not require insulin. --- ### **High-Yield Clinical Pearls for INI-CET** * **GLUT-4 Locations:** Remember the "Big Three": **Skeletal muscle**, **Cardiac muscle**, and **Adipose tissue**. * **Exercise Exception:** During exercise, skeletal muscle can take up glucose **independently of insulin** because muscle contraction triggers GLUT-4 translocation via the **AMPK pathway**. This is why exercise is a key management strategy for Type 2 Diabetes. * **GLUT-2:** Found in the **Liver, Pancreatic beta cells, and Small Intestine**. It acts as a "glucose sensor" due to its high $K_m$ (low affinity). * **SGLT:** These are **Secondary Active Transporters** (Sodium-Glucose Linked Transporters) found in the kidneys and small intestine, not to be confused with the facilitated diffusion of GLUTs.

Q: Hyperpigmentation in Addison's disease is due to increased secretion of:

B. ACTH. ***ACTH*** - In **primary adrenal insufficiency** (Addison's disease), low cortisol production results in loss of negative feedback to the pituitary gland, leading to massively increased secretion of **ACTH** (Adrenocorticotropic Hormone) [1]. - ACTH is synthesized from the precursor molecule **Pro-opiomelanocortin (POMC)**. Cleavage of POMC also generates Melanocyte-Stimulating Hormone (MSH) (or ACTH itself acts like MSH), which stimulates melanocytes, causing **hyperpigmentation** [1]. *Cortisol* - Cortisol levels are **low** in Addison's disease because of adrenal gland destruction, which is the underlying pathological issue [2]. - Cortisol is a glucocorticoid and its primary role is metabolic; it does not directly stimulate **melanogenesis**. *Aldosterone* - Aldosterone, a mineralocorticoid, is also deficient in primary Addison's disease, leading to **salt-wasting** and electrolyte disturbances (hyponatremia, hyperkalemia) [2]. - Aldosterone's synthesis pathway is distinct from that of the pituitary hormone ACTH and therefore does not influence **skin pigmentation**. *Renin* - Renin levels are often **elevated** in Addison's disease due to hypovolemia and hypotension resulting from aldosterone deficiency (activation of the **Renin-Angiotensin-Aldosterone System**) [3]. - Renin is an enzyme secreted by the kidney and has no direct mechanism or shared precursor pathway related to stimulating **melanocytes**.

Q: A woman is diagnosed with a pituitary microadenoma and has elevated serum prolactin levels. She presents with secondary amenorrhea and infertility. What is the most likely mechanism by which hyperprolactinemia causes these symptoms?

Decreased GnRH secretion from the hypothalamus. ***Decreased GnRH secretion from the hypothalamus*** - High levels of prolactin directly inhibit the pulsatile release of **Gonadotropin-Releasing Hormone (GnRH)** from the hypothalamus. - This inhibition leads to decreased pituitary secretion of **Luteinizing Hormone (LH)** and Follicle-Stimulating Hormone (FSH), causing **hypogonadotropic hypogonadism**, resulting in anovulation, amenorrhea, and infertility. *Antagonism of estrogen receptors* - Prolactin primarily exerts its reproductive effects centrally on the **hypothalamic-pituitary axis**, not by acting as a peripheral antagonist of estrogen receptors. - The resulting symptoms are due to **low estrogen production** secondary to inhibited gonadotropins, not receptor blockade. *Increased pulsatile FSH secretion* - Hyperprolactinemia actually causes **decreased** and non-pulsatile secretion of FSH and LH, rather than an increase. - If FSH were increased, it would stimulate follicular development and likely lead to ovarian hyperfunction, which is the opposite of the clinical presentation. *Increased LH secretion from the pituitary* - Prolactin actively inhibits LH release, causing **low plasma LH** levels and disrupting the mid-cycle LH surge necessary for ovulation. - The resulting state is one of inadequate follicular stimulation and anovulation, causing infertility and oligomenorrhea/amenorrhea.

Q: Which of the following is correct about the feedback hormone marked as $X$ ?

Inhibin B. **Correct: Inhibin B** - The diagram shows a feedback loop where "X" is produced downstream and inhibits the anterior pituitary. This fits the role of **Inhibin B**, which is primarily secreted by the **granulosa cells** of the ovary and **Sertoli cells** of the testis. - **Inhibin B** selectively inhibits the secretion of **FSH** from the anterior pituitary, as indicated by the negative feedback arrow pointing towards the pituitary. - Inhibin B is the key hormone in the follicular phase that provides selective FSH feedback control. *Incorrect: Inhibin A* - **Inhibin A** is predominantly secreted by the **corpus luteum** during the luteal phase of the menstrual cycle and is involved in placental function during pregnancy. - Its levels peak later in the cycle compared to Inhibin B during the follicular phase, and its primary role is not the selective inhibition of FSH shown in this diagram's context for follicular development. *Incorrect: Progesterone* - **Progesterone** is a steroid hormone primarily involved in preparing the uterus for pregnancy and maintaining it during early pregnancy. - While it exerts negative feedback on GnRH and LH/FSH, it is typically secreted by the corpus luteum after ovulation, and the diagram appears to depict a more general inhibitory feedback on FSH. *Incorrect: Estrone* - **Estrone** is one of the three major naturally occurring estrogens but is less potent than estradiol. - While estrogens provide feedback on the hypothalamus and pituitary, the diagram specifically labels "Estrogen" separately, and "X" represents a distinct feedback hormone, making estrone an unlikely specific fit for "X."

Q: The blood levels of hormones are elevated during exercise and sleep as shown. Which hormone would exhibit this diurnal pattern?

Growth hormone. ***Growth hormone*** - **Growth hormone (GH)** secretion is known to increase significantly during both **strenuous exercise** and **sleep**, particularly during deep sleep stages. - The elevated levels during exercise promote **lipolysis** and **glucose production**, while during sleep, it facilitates **tissue repair** and **growth**. *Insulin* - **Insulin** levels typically **decrease during exercise** to promote the utilization of fat as fuel and increase during sleep in response to reduced metabolic demand and preparation for morning. - Its primary role is to regulate blood glucose, and its secretion is mainly stimulated by **high blood glucose** rather than exercise or sleep directly in this pattern. *Cortisol* - **Cortisol** secretion follows a **circadian rhythm**, peaking in the early morning and gradually decreasing throughout the day, reaching its lowest point at night. - While exercise can acutely increase cortisol, its **sleep-related pattern** is the opposite of what is shown, typically decreasing during early sleep. *Thyroid* - **Thyroid hormones (T3 and T4)** maintain a relatively **stable level** throughout the day and night, with minor diurnal fluctuations. - Their primary function is to regulate **metabolism** and they do not exhibit sharp, distinct peaks in response to exercise or sleep in the manner depicted.

Question 1

What is the storage form of thyroid hormone?

Accepted Answer

Thyroglobulin

Answer

Tri-iodotyrosine

Answer

Tri-iodothyronine

Answer

Di-iodotyrosine

Question 2

Which of the following iodinated compounds is present in the maximum concentration in the thyroid?

Accepted Answer

Diiodotyrosine (DIT)

Answer

Monoiodotyrosine (MIT)

Answer

Triiodothyronine (T3)

Answer

Reverse T3

Question 3

Which of the following hormones is released from the hypothalamus?

Accepted Answer

Cortisol releasing hormone

Answer

Orexin

Answer

Neuropeptide

Answer

Ghrelin

Question 4

Which of the following is an orexigenic hormone?

Accepted Answer

Ghrelin

Answer

Melanocyte-stimulating hormone

Answer

Cholecystokinin

Answer

Leptin

Question 5

Which of the following mechanisms is used by the marked structure to influence neighboring cells?

Accepted Answer

Paracrine

Answer

Nerve mediated

Answer

Vein mediated

Answer

Autocrine

Question 6

Which of the following tissues will not be able to take up glucose in insulin resistance/insulin absence/diabetes mellitus?

Accepted Answer

Skeletal muscle

Answer

Kidney

Answer

Brain

Answer

Red blood cells

Question 7

Hyperpigmentation in Addison's disease is due to increased secretion of:

Accepted Answer

B. ACTH

Answer

C. Aldosterone

Answer

D. Renin

Answer

A. Cortisol

Question 8

A woman is diagnosed with a pituitary microadenoma and has elevated serum prolactin levels. She presents with secondary amenorrhea and infertility. What is the most likely mechanism by which hyperprolactinemia causes these symptoms?

Accepted Answer

Decreased GnRH secretion from the hypothalamus

Answer

Increased LH secretion from the pituitary

Answer

Antagonism of estrogen receptors

Answer

Increased pulsatile FSH secretion

Question 9

Which of the following is correct about the feedback hormone marked as $X$ ?

Accepted Answer

Inhibin B

Answer

Inhibin A

Answer

Progesterone

Answer

Estrone

Question 10

The blood levels of hormones are elevated during exercise and sleep as shown. Which hormone would exhibit this diurnal pattern?

Accepted Answer

Growth hormone

Answer

Insulin

Answer

Cortisol

Answer

Thyroid hormones

Endocrinology — MCQs

Endocrinology — MCQs

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