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Diagnostic Tests – Pro-Insulin
1. What is this test for?
The pro-insulin assay is used to help determine the cause of hypoglycemia, specifically to differentiate endogenous insulin overproduction from other etiologies. It is particularly useful for distinguishing insulinoma from autoimmune or exogenous causes of hypoglycemia.
2. What do high levels of pro-insulin represent?
Elevated pro-insulin levels strongly suggest the presence of an insulinoma. Insulinomas secrete excessive amounts of insulin and incompletely processed insulin precursors, leading to disproportionately high pro-insulin concentrations compared with normal physiology.
3. When do you answer pro-insulin levels?
You should choose a pro-insulin level in cases of recurrent fasting hypoglycemia, especially when the patient presents with symptoms such as weakness, confusion, diaphoresis, or tremors and laboratory findings show inappropriately elevated C-peptide levels. This combination indicates endogenous insulin secretion and points toward insulinoma as the underlying diagnosis rather than surreptitious insulin use or autoimmune hypoglycemia.
1. What is this test for?
The pro-insulin assay is used to help determine the cause of hypoglycemia, specifically to differentiate endogenous insulin overproduction from other etiologies. It is particularly useful for distinguishing insulinoma from autoimmune or exogenous causes of hypoglycemia.
2. What do high levels of pro-insulin represent?
Elevated pro-insulin levels strongly suggest the presence of an insulinoma. Insulinomas secrete excessive amounts of insulin and incompletely processed insulin precursors, leading to disproportionately high pro-insulin concentrations compared with normal physiology.
3. When do you answer pro-insulin levels?
You should choose a pro-insulin level in cases of recurrent fasting hypoglycemia, especially when the patient presents with symptoms such as weakness, confusion, diaphoresis, or tremors and laboratory findings show inappropriately elevated C-peptide levels. This combination indicates endogenous insulin secretion and points toward insulinoma as the underlying diagnosis rather than surreptitious insulin use or autoimmune hypoglycemia.
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Diagnostic Tests – ACTH (Cosyntropin) Stimulation Test
1. What is this test?
The ACTH (cosyntropin) stimulation test is a diagnostic evaluation of adrenal gland function. It is considered the gold-standard test for identifying adrenal insufficiency. The test begins with measurement of baseline serum cortisol levels. Synthetic ACTH (cosyntropin) is then administered, followed by repeat cortisol measurements at specified intervals. An inadequate cortisol response suggests adrenal insufficiency. Persistently low cortisol levels point toward primary adrenal insufficiency, whereas an appropriate rise in cortisol indicates that the adrenal glands are capable of responding to ACTH stimulation.
2. When should you order or interpret an ACTH stimulation test?
This test should be considered in patients with clinical features suggestive of adrenal insufficiency. Classic findings include chronic fatigue, generalized weakness, skin hyperpigmentation, hyperkalemia, mild metabolic acidosis, and salt craving (often described as a preference for salty foods such as pickle juice). It is also particularly important in critically ill or ICU patients who present with unexplained hypotension that does not respond adequately to fluids or vasopressors, where occult adrenal insufficiency may be life-threatening.
1. What is this test?
The ACTH (cosyntropin) stimulation test is a diagnostic evaluation of adrenal gland function. It is considered the gold-standard test for identifying adrenal insufficiency. The test begins with measurement of baseline serum cortisol levels. Synthetic ACTH (cosyntropin) is then administered, followed by repeat cortisol measurements at specified intervals. An inadequate cortisol response suggests adrenal insufficiency. Persistently low cortisol levels point toward primary adrenal insufficiency, whereas an appropriate rise in cortisol indicates that the adrenal glands are capable of responding to ACTH stimulation.
2. When should you order or interpret an ACTH stimulation test?
This test should be considered in patients with clinical features suggestive of adrenal insufficiency. Classic findings include chronic fatigue, generalized weakness, skin hyperpigmentation, hyperkalemia, mild metabolic acidosis, and salt craving (often described as a preference for salty foods such as pickle juice). It is also particularly important in critically ill or ICU patients who present with unexplained hypotension that does not respond adequately to fluids or vasopressors, where occult adrenal insufficiency may be life-threatening.
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Diagnostic Tests – Antimicrosomal Antibodies, Antithyroid Peroxidase (anti-TPO) Antibodies, and Antithyroglobulin Antibodies
1. What are they?
Antimicrosomal antibodies (now more commonly referred to as anti-thyroid peroxidase or anti-TPO antibodies) and antithyroglobulin antibodies are autoimmune markers directed against components of the thyroid gland. Their presence reflects immune-mediated damage to thyroid tissue and is classically associated with Hashimoto’s thyroiditis and Graves’ disease. In patients who are hypothyroid, a positive antibody test confirms an autoimmune cause, allowing clinicians to proceed directly with thyroid hormone replacement (e.g., levothyroxine/Synthroid) without further etiologic workup.
2. When are they the answer?
These antibody tests are used after thyroid dysfunction has already been established by abnormal thyroid function tests (such as TSH and free T4). They are not intended to diagnose whether a patient is hypo- or hyperthyroid. Instead, their role is to confirm that the underlying cause of the thyroid abnormality is autoimmune in nature, whether the patient presents with hypothyroidism or hyperthyroidism.
3. Do you expect high or low values?
Antibody levels are expected to be elevated in autoimmune thyroid disease, regardless of whether the patient is currently hypo- or hyperfunctioning. In contrast, thyroid dysfunction due to non-autoimmune causes—such as nutritional deficiencies or inappropriate use or abuse of thyroid hormone (Synthroid)—is typically associated with low or absent antibody levels.
1. What are they?
Antimicrosomal antibodies (now more commonly referred to as anti-thyroid peroxidase or anti-TPO antibodies) and antithyroglobulin antibodies are autoimmune markers directed against components of the thyroid gland. Their presence reflects immune-mediated damage to thyroid tissue and is classically associated with Hashimoto’s thyroiditis and Graves’ disease. In patients who are hypothyroid, a positive antibody test confirms an autoimmune cause, allowing clinicians to proceed directly with thyroid hormone replacement (e.g., levothyroxine/Synthroid) without further etiologic workup.
2. When are they the answer?
These antibody tests are used after thyroid dysfunction has already been established by abnormal thyroid function tests (such as TSH and free T4). They are not intended to diagnose whether a patient is hypo- or hyperthyroid. Instead, their role is to confirm that the underlying cause of the thyroid abnormality is autoimmune in nature, whether the patient presents with hypothyroidism or hyperthyroidism.
3. Do you expect high or low values?
Antibody levels are expected to be elevated in autoimmune thyroid disease, regardless of whether the patient is currently hypo- or hyperfunctioning. In contrast, thyroid dysfunction due to non-autoimmune causes—such as nutritional deficiencies or inappropriate use or abuse of thyroid hormone (Synthroid)—is typically associated with low or absent antibody levels.
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Diagnostic Tests – 24-Hour Urine for Metanephrines, Vanillylmandelic Acid (VMA), and Catecholamines
1. For which condition is this the best initial test?
A 24-hour urine collection for metanephrines, VMA, and catecholamines is the best initial screening test for pheochromocytoma. This tumor secretes excessive catecholamines, and their metabolites are more reliably detected over a full 24-hour period than with spot measurements.
2. If the test is positive, what is the next best imaging study?
If urinary metanephrines, VMA, or catecholamines are elevated, the next step is localization of the tumor. The best initial imaging studies are a CT scan or MRI of the adrenal glands, which can identify most adrenal pheochromocytomas.
3. If all imaging studies are negative, what should be done next?
If CT and MRI do not reveal a tumor, the next diagnostic step is metaiodobenzylguanidine (MIBG) scintigraphy. MIBG is a norepinephrine analog that is selectively taken up by catecholamine-secreting cells, making it useful for detecting occult, extra-adrenal, or metastatic pheochromocytomas.
4. This disease is associated with which syndrome?
Pheochromocytoma is classically associated with multiple endocrine neoplasia (MEN) syndromes, particularly MEN type II and MEN type III, and should raise suspicion for an underlying genetic endocrine disorder.
5. When do you answer VMA, metanephrines, or catecholamines?
These tests are indicated when a patient presents with episodic or paroxysmal hypertension, especially when accompanied by symptoms such as flushing, diarrhea, headaches, palpitations, or diaphoresis. The intermittent nature of hypertension and autonomic symptoms is a key clinical clue pointing toward a catecholamine-secreting tumor.
1. For which condition is this the best initial test?
A 24-hour urine collection for metanephrines, VMA, and catecholamines is the best initial screening test for pheochromocytoma. This tumor secretes excessive catecholamines, and their metabolites are more reliably detected over a full 24-hour period than with spot measurements.
2. If the test is positive, what is the next best imaging study?
If urinary metanephrines, VMA, or catecholamines are elevated, the next step is localization of the tumor. The best initial imaging studies are a CT scan or MRI of the adrenal glands, which can identify most adrenal pheochromocytomas.
3. If all imaging studies are negative, what should be done next?
If CT and MRI do not reveal a tumor, the next diagnostic step is metaiodobenzylguanidine (MIBG) scintigraphy. MIBG is a norepinephrine analog that is selectively taken up by catecholamine-secreting cells, making it useful for detecting occult, extra-adrenal, or metastatic pheochromocytomas.
4. This disease is associated with which syndrome?
Pheochromocytoma is classically associated with multiple endocrine neoplasia (MEN) syndromes, particularly MEN type II and MEN type III, and should raise suspicion for an underlying genetic endocrine disorder.
5. When do you answer VMA, metanephrines, or catecholamines?
These tests are indicated when a patient presents with episodic or paroxysmal hypertension, especially when accompanied by symptoms such as flushing, diarrhea, headaches, palpitations, or diaphoresis. The intermittent nature of hypertension and autonomic symptoms is a key clinical clue pointing toward a catecholamine-secreting tumor.
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Diagnostic Tests – 24-Hour Urine for Metanephrines, Vanillylmandelic Acid (VMA), and Catecholamines
1. For which condition is this the best initial test?
A 24-hour urine collection measuring metanephrines, vanillylmandelic acid (VMA), and catecholamines is the preferred initial screening test for pheochromocytoma. This test detects excess catecholamine production and metabolism, which is characteristic of this catecholamine-secreting tumor.
2. If the test is positive, what is the next best imaging study?
If urinary VMA, metanephrines, or catecholamines are elevated, the next step is anatomical imaging of the adrenal glands. A CT scan or MRI of the adrenal glands is performed to localize the pheochromocytoma.
3. If all standard imaging studies are negative, what should be done next?
If CT and MRI fail to identify a tumor, the next best diagnostic step is metaiodobenzylguanidine (MIBG) scintigraphy. MIBG is a norepinephrine analog that selectively accumulates in adrenal tissue and catecholamine-secreting tumors, making it useful for detecting occult or extra-adrenal pheochromocytomas.
4. This disease is associated with which syndrome?
Pheochromocytoma is classically associated with multiple endocrine neoplasia (MEN) syndromes, particularly MEN type II and MEN type III, as well as other hereditary endocrine tumor syndromes.
5. When do you answer VMA, metanephrines, or catecholamines?
These tests should be considered in patients with episodic or paroxysmal hypertension, especially when accompanied by symptoms such as headaches, palpitations, diaphoresis, flushing, or diarrhea. The episodic nature of symptoms is a key clue pointing toward a catecholamine-secreting tumor.
1. For which condition is this the best initial test?
A 24-hour urine collection measuring metanephrines, vanillylmandelic acid (VMA), and catecholamines is the preferred initial screening test for pheochromocytoma. This test detects excess catecholamine production and metabolism, which is characteristic of this catecholamine-secreting tumor.
2. If the test is positive, what is the next best imaging study?
If urinary VMA, metanephrines, or catecholamines are elevated, the next step is anatomical imaging of the adrenal glands. A CT scan or MRI of the adrenal glands is performed to localize the pheochromocytoma.
3. If all standard imaging studies are negative, what should be done next?
If CT and MRI fail to identify a tumor, the next best diagnostic step is metaiodobenzylguanidine (MIBG) scintigraphy. MIBG is a norepinephrine analog that selectively accumulates in adrenal tissue and catecholamine-secreting tumors, making it useful for detecting occult or extra-adrenal pheochromocytomas.
4. This disease is associated with which syndrome?
Pheochromocytoma is classically associated with multiple endocrine neoplasia (MEN) syndromes, particularly MEN type II and MEN type III, as well as other hereditary endocrine tumor syndromes.
5. When do you answer VMA, metanephrines, or catecholamines?
These tests should be considered in patients with episodic or paroxysmal hypertension, especially when accompanied by symptoms such as headaches, palpitations, diaphoresis, flushing, or diarrhea. The episodic nature of symptoms is a key clue pointing toward a catecholamine-secreting tumor.
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Diagnostic Tests – Venous Blood Gas (VBG)
1. When do you answer a VBG?
A venous blood gas (VBG) is most commonly used for the monitoring and follow-up of diabetic ketoacidosis (DKA). In this setting, a VBG can effectively replace an arterial blood gas (ABG) when the clinical focus is on assessing pH and carbon dioxide levels to track the severity and resolution of metabolic acidosis.
Because oxygenation (pO₂) is not required for routine DKA management, a VBG provides sufficient information while avoiding the discomfort and complications associated with arterial sampling. This makes it a practical, less painful, and widely accepted alternative for serial acid–base assessment during treatment.
1. When do you answer a VBG?
A venous blood gas (VBG) is most commonly used for the monitoring and follow-up of diabetic ketoacidosis (DKA). In this setting, a VBG can effectively replace an arterial blood gas (ABG) when the clinical focus is on assessing pH and carbon dioxide levels to track the severity and resolution of metabolic acidosis.
Because oxygenation (pO₂) is not required for routine DKA management, a VBG provides sufficient information while avoiding the discomfort and complications associated with arterial sampling. This makes it a practical, less painful, and widely accepted alternative for serial acid–base assessment during treatment.
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Diagnostic Tests: 24-Hour Urine Cortisol Level
Diagnoses for Which This Is the Most Sensitive Test
The 24-hour urinary free cortisol level is the most sensitive screening test for endogenous hypercortisolism and is the best initial test for diagnosing Cushing’s syndrome. Because cortisol secretion is variable and follows a diurnal rhythm, a 24-hour urine collection provides an integrated measurement of cortisol production over an entire day, making it more reliable than a single serum cortisol level.
What This Substance Is
Cortisol is a glucocorticoid hormone produced by the adrenal cortex. It plays a key role in metabolism, immune regulation, blood pressure control, and the physiologic response to stress. Excess cortisol leads to the characteristic physical and metabolic abnormalities seen in Cushing’s syndrome.
When to Answer Urinary Free Cortisol
You should choose 24-hour urinary free cortisol when the clinical presentation suggests Cushing’s syndrome. Classic features include:
In exam questions, the correct best initial test for suspected Cushing’s syndrome is either 24-hour urine cortisol or the overnight dexamethasone suppression test.
Diagnoses for Which This Is the Most Sensitive Test
The 24-hour urinary free cortisol level is the most sensitive screening test for endogenous hypercortisolism and is the best initial test for diagnosing Cushing’s syndrome. Because cortisol secretion is variable and follows a diurnal rhythm, a 24-hour urine collection provides an integrated measurement of cortisol production over an entire day, making it more reliable than a single serum cortisol level.
What This Substance Is
Cortisol is a glucocorticoid hormone produced by the adrenal cortex. It plays a key role in metabolism, immune regulation, blood pressure control, and the physiologic response to stress. Excess cortisol leads to the characteristic physical and metabolic abnormalities seen in Cushing’s syndrome.
When to Answer Urinary Free Cortisol
You should choose 24-hour urinary free cortisol when the clinical presentation suggests Cushing’s syndrome. Classic features include:
- Truncal (central) obesity
- “Buffalo hump”
- Purple striae
- Easy bruising
- Proximal muscle weakness
- Hypertension or glucose intolerance
In exam questions, the correct best initial test for suspected Cushing’s syndrome is either 24-hour urine cortisol or the overnight dexamethasone suppression test.
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Diagnostic Tests: Beta–Human Chorionic Gonadotropin (β-hCG)
Overview — What This Test Is
Beta–human chorionic gonadotropin (β-hCG) is a hormone produced by the placenta shortly after implantation. Measurement of β-hCG in urine or serum is used to detect pregnancy. Serum testing is more sensitive than urine testing and allows for quantitative measurement, which is useful for monitoring early pregnancy progression and identifying abnormal pregnancies.
When β-hCG Is the Correct Answer
β-hCG is the best initial diagnostic test for any woman of reproductive age who presents with symptoms that could indicate pregnancy or pregnancy-related complications. You should answer β-hCG when the clinical scenario includes:
- Amenorrhea or a missed/delayed period
- Vaginal bleeding
- Lower abdominal or pelvic pain
- Unexplained abdominal pain in a woman of childbearing age
A key exam principle is that any unexplained abdominal pain in a woman of reproductive age should be considered an ectopic pregnancy until proven otherwise, and β-hCG must be obtained first before further imaging or intervention.
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Diagnostic Tests: C-Peptide
Overview — What This Test Is
The C-peptide test measures the serum level of C-peptide, a byproduct released when proinsulin is cleaved into insulin within pancreatic β-cells. Because C-peptide is secreted only with endogenous insulin and is absent in injected (exogenous) insulin, it is the best test for distinguishing endogenous from exogenous hyperinsulinemia in patients presenting with hypoglycemia.
Interpretation of Low C-Peptide Levels
Low C-peptide levels in the setting of hypoglycemia indicate exogenous insulin administration. In this situation, insulin levels are elevated, but C-peptide is suppressed because the insulin is being injected rather than produced by the pancreas. This finding is classic for factitious hypoglycemia, often due to insulin abuse, and may be seen in patients with psychiatric disease or repeated unexplained hospital visits.
Interpretation of High C-Peptide Levels
High C-peptide levels reflect endogenous insulin production. This occurs when the pancreas is actively secreting insulin, such as in:
In these cases, both insulin and C-peptide levels are elevated during episodes of hypoglycemia.
When to Choose a C-Peptide Level
You should answer C-peptide level when a patient presents with:
In exam questions, C-peptide is the next best diagnostic step to determine the cause of hyperinsulinemic hypoglycemia and to differentiate insulin abuse from endogenous insulin excess.
Overview — What This Test Is
The C-peptide test measures the serum level of C-peptide, a byproduct released when proinsulin is cleaved into insulin within pancreatic β-cells. Because C-peptide is secreted only with endogenous insulin and is absent in injected (exogenous) insulin, it is the best test for distinguishing endogenous from exogenous hyperinsulinemia in patients presenting with hypoglycemia.
Interpretation of Low C-Peptide Levels
Low C-peptide levels in the setting of hypoglycemia indicate exogenous insulin administration. In this situation, insulin levels are elevated, but C-peptide is suppressed because the insulin is being injected rather than produced by the pancreas. This finding is classic for factitious hypoglycemia, often due to insulin abuse, and may be seen in patients with psychiatric disease or repeated unexplained hospital visits.
Interpretation of High C-Peptide Levels
High C-peptide levels reflect endogenous insulin production. This occurs when the pancreas is actively secreting insulin, such as in:
- Insulinoma
- Sulfonylurea use, which stimulates pancreatic insulin release
In these cases, both insulin and C-peptide levels are elevated during episodes of hypoglycemia.
When to Choose a C-Peptide Level
You should answer C-peptide level when a patient presents with:
- Unexplained hypoglycemia
- Elevated insulin levels
- Recurrent or unexplained hospital admissions
- Suspicion of factitious disorder or insulin-secreting tumor
In exam questions, C-peptide is the next best diagnostic step to determine the cause of hyperinsulinemic hypoglycemia and to differentiate insulin abuse from endogenous insulin excess.
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Diagnostic Tests: TSH, Thyroxine (T4), and Thyroglobulin
Clinical Scenarios for Which These Tests Are Used
TSH and thyroxine (T4) are used to evaluate thyroid function, including both hypothyroidism and hyperthyroidism. They are the cornerstone tests for assessing whether the thyroid gland is underactive or overactive and for determining whether dysfunction is primary (thyroid-related) or secondary (pituitary-related).
Thyroglobulin, in contrast, is not a test of thyroid function. It is used specifically to monitor disease recurrence or treatment response in patients who have been treated for papillary or follicular thyroid cancer.
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What These Substances Are
Thyroid-stimulating hormone (TSH) is produced by the anterior pituitary gland and regulates thyroid hormone production by stimulating the thyroid gland.
Thyroxine (T4) is one of the principal hormones produced by the thyroid gland and plays a key role in regulating metabolism, growth, and development. It is synthesized in the thyroid and later converted to the more active triiodothyronine (T3) in peripheral tissues.
Thyroglobulin is an iodine-containing glycoprotein synthesized by thyroid follicular cells. It serves as the storage form and precursor for thyroid hormone production within the thyroid gland. It should not be confused with thyroid-binding globulin, which is a plasma transport protein.
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When to Answer TSH and Thyroxine (T4)
TSH and T4 are always the best initial tests when evaluating any suspected thyroid disorder. You should choose these tests when a patient presents with symptoms suggestive of:
• Hypothyroidism (fatigue, weight gain, cold intolerance, constipation)
• Hyperthyroidism (weight loss, heat intolerance, tremor, palpitations)
• Goiter or thyroid nodules
• Abnormal vital signs or unexplained metabolic changes
Together, TSH and T4 provide the most reliable initial assessment of thyroid status.
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When to Answer Thyroglobulin
Thyroglobulin should be selected when the clinical question involves follow-up or surveillance of differentiated thyroid cancer, specifically:
• Papillary thyroid carcinoma
• Follicular thyroid carcinoma
After thyroidectomy and radioactive iodine ablation, thyroglobulin levels should be undetectable or very low. A rising thyroglobulin level suggests recurrent or residual thyroid cancer. It is not used to diagnose thyroid cancer initially and not used to assess thyroid function.
Clinical Scenarios for Which These Tests Are Used
TSH and thyroxine (T4) are used to evaluate thyroid function, including both hypothyroidism and hyperthyroidism. They are the cornerstone tests for assessing whether the thyroid gland is underactive or overactive and for determining whether dysfunction is primary (thyroid-related) or secondary (pituitary-related).
Thyroglobulin, in contrast, is not a test of thyroid function. It is used specifically to monitor disease recurrence or treatment response in patients who have been treated for papillary or follicular thyroid cancer.
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What These Substances Are
Thyroid-stimulating hormone (TSH) is produced by the anterior pituitary gland and regulates thyroid hormone production by stimulating the thyroid gland.
Thyroxine (T4) is one of the principal hormones produced by the thyroid gland and plays a key role in regulating metabolism, growth, and development. It is synthesized in the thyroid and later converted to the more active triiodothyronine (T3) in peripheral tissues.
Thyroglobulin is an iodine-containing glycoprotein synthesized by thyroid follicular cells. It serves as the storage form and precursor for thyroid hormone production within the thyroid gland. It should not be confused with thyroid-binding globulin, which is a plasma transport protein.
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When to Answer TSH and Thyroxine (T4)
TSH and T4 are always the best initial tests when evaluating any suspected thyroid disorder. You should choose these tests when a patient presents with symptoms suggestive of:
• Hypothyroidism (fatigue, weight gain, cold intolerance, constipation)
• Hyperthyroidism (weight loss, heat intolerance, tremor, palpitations)
• Goiter or thyroid nodules
• Abnormal vital signs or unexplained metabolic changes
Together, TSH and T4 provide the most reliable initial assessment of thyroid status.
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When to Answer Thyroglobulin
Thyroglobulin should be selected when the clinical question involves follow-up or surveillance of differentiated thyroid cancer, specifically:
• Papillary thyroid carcinoma
• Follicular thyroid carcinoma
After thyroidectomy and radioactive iodine ablation, thyroglobulin levels should be undetectable or very low. A rising thyroglobulin level suggests recurrent or residual thyroid cancer. It is not used to diagnose thyroid cancer initially and not used to assess thyroid function.
