Measuring hormone levels
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Measuring hormone levels
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[post_date] => 2024-12-23 18:18:54
[post_date_gmt] => 2024-12-23 23:18:54
[post_content] => Practice Passage (Question 1-6)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Most hormones are present in the circulating blood in extremely low concentrations, some as low as one millionth of a microgram (one picogram) per mL. This made measuring hormone concentrations difficult in early endocrinology. In the mid-20th century, however, a revolutionary technique called radioimmunoassay was developed, making it possible to measure hormones and their end products.
Radioimmunoassay occurs in several steps. First, an antibody specific for a hormone is mixed with a sample of purified hormone of known concentration that has been labeled with a radioactive isotope. Second, a sample of fluid from the organism to be tested is also added to this mixture. It is important to note that the amount of the antibody used has to be less than the amount needed to bind to the combined quantity of hormone in the fluid. This antibody deficiency creates competition among antibody binding sites (Step 3), and is the basis upon which the relative concentration of each sample can be determined.
In the final step, unbound antigens are removed from the mixture, and radioactive bound antigens are measured. Results of these experiments are compared to a “standard curve,” indicating the relationship between radioactive concentrations and organism hormone concentrations.
The method is summarized in Figure 1.
Figure 1 Radioimmunoassay; image adapted from Howell-Moroney, Madeleine, "Radioimmunoassay - Image". Embryo Project Encyclopedia ( 2022-01-24 ). ISSN: 1940-5030 https://hdl.handle.net/10776/13324
When compared with test assays, “standard curves” of hormone concentrations can elucidate the unknown concentrations within an error of 10-15 percent, allowing the assay of billionths or even trillionths of a gram of hormone.
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[question] => The standard RIA (radioimmunoassay) curve of percent-radioactive binding as a function of hormone concentration would most resemble which plot?
[value] => Array
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[answer] => 3
[description] => Reason for Correct Answer:
Hormone concentration is the independent variable and would be represented by the x-axis. Alternatively, percent-radioactive binding represents the dependent variable and is represented by the y-axis.
The organism’s hormone competes with the radioactive hormone for binding to antibodies.
A large amount of radioactive hormone binding is therefore indicative of a relatively small hormone concentration in the organism’s fluid and vice versa.
The third graph shows the correct relationship between hormone concentration and percent radioactive binding. Initially, hormone concentration is low and radioactivity is high, but radioactivity decreases as hormone concentration is elevated.
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[answers] => Array
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[0] => Array
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[each_answer] => A. 
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[1] => Array
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[each_answer] => B.
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[2] => Array
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[each_answer] => C.
)
[3] => Array
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[each_answer] => D.
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[1] => Array
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[quiz_unique_key] => 1403770772
[question] => Increased production of cortisol in response to stress would most likely accompany increased:
[value] => Array
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[answer] => 4
[description] => Reason for Correct Answer:
Cortisol is produced in the cortex (outer layer) of the adrenal gland which is situated atop the kidneys.
The corticotropin-releasing hormone (CRH) from the hypothalamus triggers adrenocorticotropic hormone (ACTH) from the anterior pituitary to be produced, and this in turn stimulates cortisol production in the adrenal glands.
Cortisol has many effects on the body. One important effect is that it increases blood pressure by increasing the sensitivity of the vasculature to epinephrine and norepinephrine.
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[answers] => Array
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[0] => Array
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[each_answer] => A. secretion of digestive enzymes.
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[1] => Array
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[each_answer] => B. constriction of pupils.
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[each_answer] => C. bone formation.
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[each_answer] => D. blood pressure.
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[2] => Array
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[quiz_unique_key] => 1403770772
[question] => Low serum levels of iodine, a necessary component in the production of thyroid hormones, would lead to which of the following?
I. Goiter
II. Hypothyroidism
III. Elevated serum TSH
[value] => Array
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[answer] => 4
[description] => Reason for Correct Answer:
The question stem states that iodine is necessary in the production of thyroid hormones. Insufficient levels of iodine would result in decreased thyroid hormone production.
When thyroid hormone levels are low, the thyroid gland is stimulated to increase production by the anterior pituitary hormone TSH (thyroid stimulating hormone).
Hypothyroidism is a medical condition characterized by low circulating levels of thyroid hormones; these low levels could result from low iodine levels.
When thyroid hormone levels are low, the increased TSH continues to stimulate the thyroid gland to increase production of its hormones. This can result in thyroid tissue growth (goiter) to compensate for decreased efficacy.
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[each_answer] => A. I only
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[each_answer] => B. II only
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[each_answer] => C. I and II only
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[each_answer] => D. I, II, and III
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[quiz_unique_key] => 1403770772
[question] => An RIA (radioimmunoassay) for cortisol, performed on a healthy adult in the afternoon yielded a cortisol concentration of 8 μg/dL. If the adult’s blood was taken during a traumatic event, one would expect the radioactivity measured in step 4 to:
[value] => Array
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[answer] => 3
[description] => Reason for Correct Answer:
Cortisol is a stress hormone.
Traumatic events often initiate a stress response.
During a traumatic event, cortisol levels would rise in conjunction with the body’s stress response.
This would result in a decrease in radioactivity in Step 4, since there would be more unlabeled (natural) cortisol from the patient to compete for binding with the radioactive “antigen” (which would be radio-labeled cortisol in this assay).
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[answers] => Array
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[0] => Array
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[each_answer] => A. increase significantly.
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[1] => Array
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[each_answer] => B. increase slightly.
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[2] => Array
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[each_answer] => C. decrease.
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[3] => Array
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[each_answer] => D. remain unchanged.
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[quiz_unique_key] => 1403770772
[question] => A researcher uses the radioimmunoassay to measure cortisol levels in a rat. The level of radioactive binding is so low that it does not fall on the standard curve that the researcher created to measure concentration. How should the researcher determine the correct cortisol level in the rat?
[value] => Array
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[answer] => 2
[description] => Reason for Correct Answer:
Standard curves are important tools for identifying characteristics of unknown samples. Standard curves are created with known quantities of material and are not accurate if extrapolated out since the relationships between variables are not known beyond the plotted curve.
The passage says that in this case, the more hormone that is present in the fluid, the lower the level of radioactive binding.
The levels of radioactive binding are too low to detect in this assay, therefore the hormone levels in the fluid are too high to measure.
Diluting the hormone until it falls on the standard curve in this assay is the best way to determine the hormone concentration. The resulting concentration can be multiplied by the dilution factor to determine actual concentration.
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[answers] => Array
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[each_answer] => A. Extrapolate out the standard curve until the level of radioactive binding falls on the graph.
)
[1] => Array
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[each_answer] => B. Dilute the hormone, redo the assay, and determine if the level of radioactive binding falls on the standard curve. The resulting concentration can then be multiplied by the dilution factor to determine the actual concentration.
)
[2] => Array
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[each_answer] => C. Identify a different rat in which to perform the assay since the fluid in this rat contains hormone levels that are too high to measure.
)
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[each_answer] => D. Identify a different rat in which to perform the assay since the fluid in this rat contains hormone levels that are too low to measure.
)
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[quiz_unique_key] => 1325138223
[question] => **If RIA was able to identify one trillionth of a gram of Hormone 1 but only one billionth of a gram of Hormone 2, what would be the respective limits of detection for each hormone?
[value] => Array
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[answer] => 1
[description] => Reason for Correct Answer:
1 billion is 1,000,000,000 or 10⁹ 1 trillion is 1,000,000,000,000 or 10¹²
1 billionth of a gram, the detection limit for Hormone 1, is therefore 1/10⁹ g = 10⁻⁹ g.
1 trillionth of a gram, the detection limit for Hormone 2, is therefore 1/10¹² g = 10⁻¹² g.
10⁻⁹ g is a nanogram, or ng. So, the limit of detection for Hormone 1 is 1 ng.
10⁻¹² g, the limit of detection for Hormone 2, is a picrogram (pg). But, represented in terms of nanograms, it equals:
10⁻¹² g / [10⁻⁹ g/ng] = 10⁻³ ng = 0.001 ng
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[answers] => Array
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[each_answer] => A. 1 ng and 0.001 ng
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[each_answer] => B. 1 ng and 0.0001 ng
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[each_answer] => C. 1 μg and 0.001 μg
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[each_answer] => D. 1 μg and 0.0001 μg
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[554651|4] => C
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Figure 1
