Thyroid Physiology — MCQs
Regarding thyroid hormone all are true except:
In a pregnant woman with hyperemesis gravidarum and abnormal thyroid function tests showing hyperthyroidism, which hormone is likely elevated?
A 20-year-old male presents with weight loss, heat intolerance, bilateral exophthalmos, a lid lag, sweating, and tachycardia. These symptoms are due to an increased production and secretion of a hormone that is derived from which one of the following?
All the following hormones have receptors on the plasma membrane of target tissues except:
A 30-year-old female complaints of fatigue and is unable to gain weight. On examination, her body felt warm. Which of the following investigation can be helpful in reaching the diagnosis?
Secretion of prolactin is inhibited by?
Which of the following is FALSE about Ghrelin?
Which of the following causes hypocalcemia:
If iodine supplementation in the diet was completely stopped today, thyroid hormone levels in blood will be severely depleted after:
Thyroid hormone binds to which receptor ?
Thyroid Physiology Indian Medical PG Practice Questions and MCQs
Question 1: Regarding thyroid hormone all are true except:
- A. T4 has the maximum plasma concentration
- B. T3 is more avidly bound to nuclear receptors than T4
- C. T3 is more active than T4
- D. T4 has shorter half life than T3 (Correct Answer)
Explanation: ***T4 has shorter half-life than T3*** - This statement is incorrect because **T4 (thyroxine) has a significantly longer half-life (approximately 7 days)** compared to **T3 (triiodothyronine), which has a half-life of about 1-2 days**. - The longer half-life of T4 allows for a more stable and sustained effect, acting as a prohormone. *T4 has the maximum plasma concentration* - **T4 is secreted in much larger quantities from the thyroid gland than T3 (about 80% T4 vs. 20% T3)**, leading to a higher concentration in the plasma. - This high plasma concentration of T4 makes it the primary circulating thyroid hormone, mostly bound to plasma proteins. *T3 is more avidly bound to nuclear receptors than T4* - **T3 binds to nuclear thyroid hormone receptors with 10 to 15 times greater affinity than T4**, making it the more potent and biologically active form. - This stronger binding affinity is crucial for T3's direct physiological effects on target cells. *T3 is more active than T4* - **T3 is considered the metabolically active form of thyroid hormone**, directly mediating most physiological effects by binding to nuclear receptors. - T4 acts largely as a **prohormone**, being deiodinated in peripheral tissues to form T3, which then exerts metabolic activity.
Question 2: In a pregnant woman with hyperemesis gravidarum and abnormal thyroid function tests showing hyperthyroidism, which hormone is likely elevated?
- A. Estradiol
- B. hCG (Correct Answer)
- C. Progesterone
- D. TSH
Explanation: ***hCG*** - Elevated levels of **human chorionic gonadotropin (hCG)** are strongly associated with **hyperemesis gravidarum** due to its structural similarity to **TSH**. - High hCG can bind to TSH receptors in the thyroid gland, leading to transient but significant **hyperthyroidism**. *Estradiol* - Estrogen levels do increase throughout pregnancy, but elevated **estradiol** is not primarily implicated in the direct cause of hyperemesis gravidarum or the associated transient hyperthyroidism. - While it contributes to pregnancy physiology, there's no direct pathway linking high estradiol to thyroid dysfunction in this context. *Progesterone* - **Progesterone** levels rise steadily during pregnancy to maintain the uterine lining and prevent contractions. - However, progesterone does not directly cause hyperemesis gravidarum or the thyroid function test abnormalities seen in this condition. *TSH* - In a state of **hyperthyroidism**, as suggested by positive thyroid function tests, **TSH (thyroid-stimulating hormone)** levels would typically be **suppressed or low**, not elevated. - The high hCG acts as a TSH mimetic, stimulating the thyroid directly and hence reducing pituitary TSH secretion.
Question 3: A 20-year-old male presents with weight loss, heat intolerance, bilateral exophthalmos, a lid lag, sweating, and tachycardia. These symptoms are due to an increased production and secretion of a hormone that is derived from which one of the following?
- A. Tryptophan
- B. Dopamine
- C. Cholesterol
- D. Tyrosine (Correct Answer)
Explanation: ***Tyrosine*** - The symptoms described (weight loss, heat intolerance, exophthalmos, lid lag, sweating, tachycardia) are characteristic of **hyperthyroidism**, specifically **Graves' disease**. - Thyroid hormones (**T3 and T4**) are synthesized from the amino acid **tyrosine** through iodination of tyrosine residues on thyroglobulin in the thyroid gland. - This makes tyrosine the direct precursor for thyroid hormone synthesis. *Tryptophan* - **Tryptophan** is a precursor for the synthesis of **serotonin** and **melatonin**, neurotransmitters involved in mood regulation and sleep-wake cycles, not thyroid function. - Deficiency or altered metabolism of tryptophan is associated with conditions like **depression** or **carcinoid syndrome**, which have different clinical presentations. *Dopamine* - **Dopamine** is a catecholamine neurotransmitter, not a precursor for thyroid hormones. - While dopamine is synthesized from tyrosine (via the pathway: tyrosine → L-DOPA → dopamine), this is a completely separate biochemical pathway from thyroid hormone synthesis. - Symptoms of altered dopamine levels are associated with conditions like **Parkinson's disease** (dopamine deficiency) or **schizophrenia** (dopamine dysregulation), which do not match this patient's hyperthyroid presentation. *Cholesterol* - **Cholesterol** is the precursor for **steroid hormones** (glucocorticoids, mineralocorticoids, androgens, estrogens) and vitamin D, but not thyroid hormones. - Conditions related to altered cholesterol-derived hormone metabolism include various **endocrine disorders** involving adrenal or gonadal dysfunction, which present with different clinical features.
Question 4: All the following hormones have receptors on the plasma membrane of target tissues except:
- A. Epinephrine
- B. Glucagon
- C. Estradiol (Correct Answer)
- D. Thyrotropin
Explanation: ***Estradiol*** - **Estradiol** is a **steroid hormone** derived from cholesterol, making it **lipid-soluble**. - Due to its lipid solubility, estradiol can readily pass through the **plasma membrane** and bind to **intracellular receptors** in the cytoplasm or nucleus. *Epinephrine* - **Epinephrine** is a **catecholamine hormone** and is **water-soluble**. - Water-soluble hormones cannot cross the lipid bilayer of the plasma membrane and thus bind to **receptors located on the cell surface**. *Glucagon* - **Glucagon** is a **peptide hormone** and is **water-soluble**. - Like other peptide hormones, it binds to **specific receptors embedded in the plasma membrane** to elicit its cellular effects via second messenger systems. *Thyrotropin* - **Thyrotropin**, also known as **Thyroid-Stimulating Hormone (TSH)**, is a **glycoprotein hormone** and is **water-soluble**. - TSH exerts its action by binding to **receptors on the plasma membrane** of thyroid follicular cells.
Question 5: A 30-year-old female complaints of fatigue and is unable to gain weight. On examination, her body felt warm. Which of the following investigation can be helpful in reaching the diagnosis?
- A. Elevated TSH with normal thyroid hormone levels
- B. Elevated TSH with low thyroid hormone levels
- C. Normal TSH with abnormal thyroid hormone levels
- D. Suppressed TSH with elevated thyroid hormone levels (Correct Answer)
Explanation: Suppressed TSH with elevated thyroid hormone levels - The patient's symptoms of **fatigue** (despite being warm) and **difficulty gaining weight**, coupled with her body feeling **warm**, are classic signs of **hyperthyroidism**. [1] - In hyperthyroidism, the thyroid gland produces **excessive thyroid hormones (T3 and T4)**, which in turn **suppresses TSH** production from the pituitary gland through negative feedback. [2] *Elevated TSH with normal thyroid hormone levels* - This pattern is characteristic of **subclinical hypothyroidism**, where the thyroid gland is beginning to fail, leading to increased TSH to maintain normal thyroid hormone levels. [3] - The patient's symptoms of feeling warm and difficulty gaining weight are inconsistent with hypothyroidism. [1] *Elevated TSH with low thyroid hormone levels* - This indicates **primary hypothyroidism**, where the thyroid gland is underactive and produces insufficient thyroid hormones, leading to a compensatory rise in TSH. [2] - Hypothyroidism typically presents with **weight gain**, **cold intolerance**, and fatigue, which contradict the patient's presentation. [1] *Normal TSH with abnormal thyroid hormone levels* - This scenario usually suggests **central hypothyroidism** (pituitary or hypothalamic dysfunction affecting TSH production) or **thyroid hormone resistance**. [3] - While possible in some rare cases, it does not fit the typical clinical picture of hyperthyroidism presented by the patient's symptoms.
Question 6: Secretion of prolactin is inhibited by?
- A. Dopamine (Correct Answer)
- B. Serotonin
- C. Noradrenaline
- D. Adrenaline
Explanation: ***Dopamine*** - **Dopamine**, produced by the **hypothalamus**, is the primary physiological inhibitor of **prolactin secretion** from the anterior pituitary gland. - It acts on **D2 receptors** on lactotrophs, leading to a decrease in prolactin synthesis and release. *Serotonin* - **Serotonin** generally has a stimulatory effect on **prolactin secretion**, rather than an inhibitory one. - Elevated serotonin levels can lead to **hyperprolactinemia**. *Noradrenaline* - While **noradrenaline** can have complex effects on pituitary hormones, it is not considered the primary direct inhibitor of **prolactin secretion**. - Its influence is often indirect or less potent than that of **dopamine**. *Adrenaline* - **Adrenaline** (epinephrine) is a neurotransmitter and hormone primarily involved in the **"fight or flight" response** and does not directly inhibit **prolactin secretion**. - Its effects on pituitary hormone release are typically less direct compared to **dopamine's** specific action on lactotrophs.
Question 7: Which of the following is FALSE about Ghrelin?
- A. Stimulates growth
- B. Produced by stomach cells
- C. Is related to regulation of thyroid hormone (Correct Answer)
- D. Increased appetite
Explanation: ***Is related to regulation of thyroid hormone*** - **Ghrelin** is primarily involved in **appetite regulation** and **growth hormone secretion**, not direct regulation of thyroid hormones. - While metabolic processes are interconnected, ghrelin does not have a direct, established role in the synthesis or release of **thyroid hormones**. *Stimulates growth* - Ghrelin is a potent **stimulator of growth hormone (GH) release** from the pituitary gland. - This action indirectly contributes to **growth processes** and metabolic regulation. *Produced by stomach cells* - The majority of ghrelin is produced by **P/D1 cells** (also known as enterochromaffin-like cells) primarily located in the **fundus of the stomach**. - Smaller amounts are also produced in the intestine, pancreas, and brain. *Increased appetite* - Ghrelin is often referred to as the "**hunger hormone**" because it stimulates appetite, promoting food intake. - Its levels typically **rise before meals** and fall after meals.
Question 8: Which of the following causes hypocalcemia:
- A. 1, 25-dihydroxycholecalciferol
- B. Parathormone
- C. Thyroid hormones
- D. Calcitonin (Correct Answer)
Explanation: ***Calcitonin*** - **Calcitonin** is a hormone secreted by the **parafollicular cells (C cells)** of the thyroid gland. - It **lowers serum calcium levels** by **inhibiting osteoclast activity** (preventing bone resorption) and **increasing renal calcium excretion**. - This is the only hormone among the options that causes hypocalcemia. *1,25-dihydroxycholecalciferol* - This is the **active form of vitamin D** (calcitriol), which **increases serum calcium levels**. - It promotes intestinal calcium absorption, enhances bone resorption, and increases renal calcium reabsorption. - Deficiency of this hormone leads to hypocalcemia, but the hormone itself raises calcium. *Parathormone* - **Parathyroid hormone (PTH)** is the primary regulator that **increases serum calcium levels**. - It stimulates **osteoclast activity** (releasing calcium from bone), increases renal calcium reabsorption, and promotes synthesis of 1,25-dihydroxycholecalciferol. - PTH acts opposite to calcitonin in calcium homeostasis. *Thyroid hormones* - **Thyroxine (T4) and triiodothyronine (T3)** primarily regulate metabolism and have **no direct role in calcium homeostasis**. - While severe thyroid dysfunction can indirectly affect bone turnover, thyroid hormones do not directly cause hypocalcemia.
Question 9: If iodine supplementation in the diet was completely stopped today, thyroid hormone levels in blood will be severely depleted after:
- A. 7 days
- B. 30 days
- C. 360 days
- D. 90 days (Correct Answer)
Explanation: *7 days* - A 7-day period is too short for thyroid hormone levels to be severely depleted, given the **large iodine reserve** in the thyroid gland. - While daily iodine intake is essential, the body's stores provide a **buffer against acute deficiencies**. *30 days* - While some changes might begin to occur after 30 days, severe depletion of **thyroid hormone levels** is unlikely as the iodine stores are still substantial. - The body's homeostatic mechanisms would continue to draw upon the **thyroid's iodine reserve** during this period. *360 days* - A 360-day period is *too long*; severe depletion of **thyroid hormone levels** would have occurred much earlier. - By this point, the individual would likely be experiencing significant symptoms of **hypothyroidism** due to chronic iodine deficiency. ***90 days*** - The human body has a **significant reserve of iodine** stored in the thyroid gland, primarily within **colloid**. - This stored iodine is sufficient to maintain normal thyroid hormone production for approximately **2 to 4 months** in the absence of dietary intake. - Therefore, **90 days (approximately 3 months)** is when thyroid hormone levels would be severely depleted after complete cessation of iodine supplementation.
Question 10: Thyroid hormone binds to which receptor ?
- A. Membrane
- B. Cytoplasmic
- C. Nuclear (Correct Answer)
- D. None of the options
Explanation: ***Nuclear*** - Thyroid hormones, being **lipid-soluble**, readily diffuse across the **cell membrane** to bind to receptors located in the nucleus. - This binding directly influences **gene expression** and protein synthesis, mediating the hormone's effects. *Membrane* - Membrane receptors typically bind **water-soluble hormones** (e.g., peptide hormones, catecholamines) that cannot freely cross the cell membrane. - These interactions usually trigger a **second messenger cascade** within the cell. *Cytoplasmic* - While some **steroid hormones** bind to cytoplasmic receptors which then translocate to the nucleus, thyroid hormones bind directly to nuclear receptors. - Cytoplasmic receptors are located in the **cytosol** before their ligand-induced translocation. *None of the options* - This option is incorrect, as thyroid hormones have a specific and well-defined receptor location. - The direct action on **gene regulation** necessitates a nuclear receptor.
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