COMSAE PHASE 1 ENDOCRINE
PHYSIOLOGY PRACTICE EXAM WITH
ACTUAL QUESTIONS AND VERIFIED
ANSWERS, PLUS EXPLAINED
RATIONALES/EXPERT VERIFIED FOR
GUARANTEED 100% PASS 2026/LATEST
UPDATE/INSTANT DOWNLOAD PDF
1. A 24-year-old woman has fatigue, weight gain, constipation, cold
intolerance, and dry skin. Laboratory studies show a low free T4
and an elevated TSH. Which physiological mechanism best
explains the elevated TSH?
A. Increased hypothalamic secretion of somatostatin
B. Reduced negative feedback of thyroid hormone on the hypothalamus
and pituitary
C. Increased conversion of T4 to reverse T3
D. Increased thyroid-binding globulin synthesis
E. Increased secretion of dopamine from the hypothalamus
Answer: B. Reduced negative feedback of thyroid hormone on the
hypothalamus and pituitary
Rationale: Primary hypothyroidism causes decreased circulating T4
and T3, reducing negative feedback at both the hypothalamus and
anterior pituitary. As a result, TRH and especially TSH secretion
increase. The elevated TSH stimulates the thyroid in an attempt to
restore thyroid hormone production. Increased TSH with low free T4
therefore indicates primary rather than central hypothyroidism.
1
, 2. A patient with hyperthyroidism has an elevated free T4 and
suppressed TSH. Which additional physiological effect of thyroid
hormone is most likely?
A. Decreased β-adrenergic receptor expression
B. Decreased basal metabolic rate
C. Increased oxygen consumption and heat production
D. Decreased intestinal glucose absorption
E. Reduced cardiac responsiveness to catecholamines
Answer: C. Increased oxygen consumption and heat production
Rationale: Thyroid hormone increases basal metabolic rate by
stimulating Na+/K+-ATPase activity, mitochondrial activity, substrate
utilization, and oxygen consumption. It also increases heat production.
Thyroid hormone enhances β-adrenergic responsiveness, contributing
to tachycardia, tremor, anxiety, and increased cardiac output in
hyperthyroidism.
3. A 45-year-old man has a pituitary macroadenoma that compresses
thyrotroph cells. Which laboratory pattern is most likely?
A. High TSH, high T4
B. High TSH, low T4
C. Low TSH, low T4
D. Low TSH, high T4
E. Normal TSH, high T4
Answer: C. Low TSH, low T4
Rationale: Pituitary failure causes secondary hypothyroidism. Because
TSH secretion is impaired, thyroid stimulation falls and circulating T4
decreases. TSH is therefore low or inappropriately normal despite low
thyroid hormone. In primary hypothyroidism, TSH would instead be
elevated.
2
, 4. Which enzyme is responsible for conversion of T4 to the more
biologically active thyroid hormone T3 in peripheral tissues?
A. Thyroid peroxidase
B. 5′-deiodinase
C. Aromatase
D. 21-hydroxylase
E. Tyrosine hydroxylase
Answer: B. 5′-deiodinase
Rationale: Type 1 and type 2 5′-deiodinases remove an iodine atom
from the outer ring of T4 to produce T3. T3 has substantially greater
biological activity than T4. Removal of an iodine from the inner ring
produces reverse T3, an inactive metabolite.
5. A patient is fasting for several days. Which change in thyroid
hormone metabolism is expected during prolonged caloric
restriction?
A. Increased conversion of T4 to T3
B. Increased reverse T3 production
C. Increased TSH secretion despite normal thyroid function
D. Increased thyroid hormone receptor sensitivity
E. Increased hepatic clearance of T3 only
Answer: B. Increased reverse T3 production
Rationale: During fasting and severe caloric restriction, peripheral
conversion of T4 shifts away from active T3 and toward inactive
reverse T3. This decreases metabolic expenditure during energy
deprivation. This physiological adaptation contributes to the euthyroid
sick syndrome pattern observed during systemic illness or starvation.
3
, 6. A 30-year-old woman has polyuria and polydipsia. Her serum
sodium is 151 mEq/L and urine osmolality is 90 mOsm/kg. After
administration of desmopressin, urine osmolality increases
dramatically. Which physiological defect is most likely?
A. Excessive aldosterone secretion
B. Resistance of collecting-duct principal cells to ADH
C. Deficient ADH secretion
D. Excessive atrial natriuretic peptide secretion
E. Increased renin secretion
Answer: C. Deficient ADH secretion
Rationale: Central diabetes insipidus results from inadequate ADH
secretion. Without ADH, collecting ducts have reduced aquaporin-2
insertion, causing impaired water reabsorption, dilute urine, and
hypernatremia. Desmopressin replaces the missing ADH effect,
producing a marked increase in urine concentration. Nephrogenic
diabetes insipidus would show little response to desmopressin.
7. ADH increases water permeability in the collecting duct primarily
by causing insertion of which channel into the apical membrane?
A. Aquaporin-1
B. Aquaporin-2
C. ENaC
D. Na+/K+-ATPase
E. ROMK
Answer: B. Aquaporin-2
Rationale: ADH binds V2 receptors on the basolateral membrane of
collecting-duct principal cells. This activates Gs, increases cAMP,
activates protein kinase A, and promotes insertion of aquaporin-2
4
PHYSIOLOGY PRACTICE EXAM WITH
ACTUAL QUESTIONS AND VERIFIED
ANSWERS, PLUS EXPLAINED
RATIONALES/EXPERT VERIFIED FOR
GUARANTEED 100% PASS 2026/LATEST
UPDATE/INSTANT DOWNLOAD PDF
1. A 24-year-old woman has fatigue, weight gain, constipation, cold
intolerance, and dry skin. Laboratory studies show a low free T4
and an elevated TSH. Which physiological mechanism best
explains the elevated TSH?
A. Increased hypothalamic secretion of somatostatin
B. Reduced negative feedback of thyroid hormone on the hypothalamus
and pituitary
C. Increased conversion of T4 to reverse T3
D. Increased thyroid-binding globulin synthesis
E. Increased secretion of dopamine from the hypothalamus
Answer: B. Reduced negative feedback of thyroid hormone on the
hypothalamus and pituitary
Rationale: Primary hypothyroidism causes decreased circulating T4
and T3, reducing negative feedback at both the hypothalamus and
anterior pituitary. As a result, TRH and especially TSH secretion
increase. The elevated TSH stimulates the thyroid in an attempt to
restore thyroid hormone production. Increased TSH with low free T4
therefore indicates primary rather than central hypothyroidism.
1
, 2. A patient with hyperthyroidism has an elevated free T4 and
suppressed TSH. Which additional physiological effect of thyroid
hormone is most likely?
A. Decreased β-adrenergic receptor expression
B. Decreased basal metabolic rate
C. Increased oxygen consumption and heat production
D. Decreased intestinal glucose absorption
E. Reduced cardiac responsiveness to catecholamines
Answer: C. Increased oxygen consumption and heat production
Rationale: Thyroid hormone increases basal metabolic rate by
stimulating Na+/K+-ATPase activity, mitochondrial activity, substrate
utilization, and oxygen consumption. It also increases heat production.
Thyroid hormone enhances β-adrenergic responsiveness, contributing
to tachycardia, tremor, anxiety, and increased cardiac output in
hyperthyroidism.
3. A 45-year-old man has a pituitary macroadenoma that compresses
thyrotroph cells. Which laboratory pattern is most likely?
A. High TSH, high T4
B. High TSH, low T4
C. Low TSH, low T4
D. Low TSH, high T4
E. Normal TSH, high T4
Answer: C. Low TSH, low T4
Rationale: Pituitary failure causes secondary hypothyroidism. Because
TSH secretion is impaired, thyroid stimulation falls and circulating T4
decreases. TSH is therefore low or inappropriately normal despite low
thyroid hormone. In primary hypothyroidism, TSH would instead be
elevated.
2
, 4. Which enzyme is responsible for conversion of T4 to the more
biologically active thyroid hormone T3 in peripheral tissues?
A. Thyroid peroxidase
B. 5′-deiodinase
C. Aromatase
D. 21-hydroxylase
E. Tyrosine hydroxylase
Answer: B. 5′-deiodinase
Rationale: Type 1 and type 2 5′-deiodinases remove an iodine atom
from the outer ring of T4 to produce T3. T3 has substantially greater
biological activity than T4. Removal of an iodine from the inner ring
produces reverse T3, an inactive metabolite.
5. A patient is fasting for several days. Which change in thyroid
hormone metabolism is expected during prolonged caloric
restriction?
A. Increased conversion of T4 to T3
B. Increased reverse T3 production
C. Increased TSH secretion despite normal thyroid function
D. Increased thyroid hormone receptor sensitivity
E. Increased hepatic clearance of T3 only
Answer: B. Increased reverse T3 production
Rationale: During fasting and severe caloric restriction, peripheral
conversion of T4 shifts away from active T3 and toward inactive
reverse T3. This decreases metabolic expenditure during energy
deprivation. This physiological adaptation contributes to the euthyroid
sick syndrome pattern observed during systemic illness or starvation.
3
, 6. A 30-year-old woman has polyuria and polydipsia. Her serum
sodium is 151 mEq/L and urine osmolality is 90 mOsm/kg. After
administration of desmopressin, urine osmolality increases
dramatically. Which physiological defect is most likely?
A. Excessive aldosterone secretion
B. Resistance of collecting-duct principal cells to ADH
C. Deficient ADH secretion
D. Excessive atrial natriuretic peptide secretion
E. Increased renin secretion
Answer: C. Deficient ADH secretion
Rationale: Central diabetes insipidus results from inadequate ADH
secretion. Without ADH, collecting ducts have reduced aquaporin-2
insertion, causing impaired water reabsorption, dilute urine, and
hypernatremia. Desmopressin replaces the missing ADH effect,
producing a marked increase in urine concentration. Nephrogenic
diabetes insipidus would show little response to desmopressin.
7. ADH increases water permeability in the collecting duct primarily
by causing insertion of which channel into the apical membrane?
A. Aquaporin-1
B. Aquaporin-2
C. ENaC
D. Na+/K+-ATPase
E. ROMK
Answer: B. Aquaporin-2
Rationale: ADH binds V2 receptors on the basolateral membrane of
collecting-duct principal cells. This activates Gs, increases cAMP,
activates protein kinase A, and promotes insertion of aquaporin-2
4
