Endocrine Disease Part 1

Question 1

What is the key function of hormones?

Answer 1

To bind a specific receptor to trigger a cascade of intracellular signaling molecules leading to a change in metabolism

Question 2

Hormones are classified by the cells they target. What are these classifications?

Answer 2

Endocrine, paracrine, autocrine, and neuroendocrine

Question 3

Hormones are classified by the cells they target. What does endocrine mean in this context?

Answer 3

The hormone is released into the blood stream to reach distant target cells

Question 4

Hormones are classified by the cells they target. What does paracrine mean in this context?

Answer 4

The hormone is released into the extracellular space and thus has its impact on neighboring cells (local action via diffusion from the source cell to the target cells separated from each other only by the extracellular space)

Question 5

Hormones are classified by the cells they target. What does autocrine mean in this context?

Answer 5

The cell producing the hormone is also the target cell

Question 6

Hormones are classified by the cells they target. What does neuroendocrine mean in this context?

Answer 6

The hormone is released from a neuron. The hormone then acts in an endocrine or paracrine manner

Question 7

______ hormones stimulate the release of _____ hormones which then stimulate the release of _____ hormones which then act on target tissues to produce an effect

Answer 7

RELEASING hormones stimulate the release of TROPIC hormones which then stimulate the release of EFFECTOR hormones which then act on target tissues to produce an effect

Question 8

Why is it important to understand the regulatory control of hormone secretion?

Answer 8

Understanding how a hormone is regulated is necessary to understand / recognize the clinical significance of basal hormone concentrations (e.g. if normal regulation results in variation of production throughout the day, basal concentrations will vary throughout the day without be of clinical significance)

Question 9

Why might an increase in hormone concentration manifest clinically as a hormone deficiency?

Answer 9

This could occur if the target receptor has developed a resistance to the hormone

Question 10

Regulatory mechanisms of hormones fall into two categories. What are these categories?

Answer 10

1. Constant secretion (constant plasma concentration)
2. Wide fluctuations in secretion (varying plasma concentration)

Question 11

What are the three types of mammalian hormones based on biochemical structure?

Answer 11

1. Peptides or peptide derivatives
2. Steroids
3. Amine hormones

Question 12

By what fluids are hormones transported through the body?

Answer 12

Lymph, extracellular fluid, and blood

Question 13

What factors affect the distribution of free and bound hormone in plasma?

Answer 13

1. The amount of hormone
2. The amount of binding protein
3. The affinity of the hormone for the protein

Question 14

What factors determine the distribution of hormones between tissue and plasma?

Answer 14

This is determined by the balance between tissue binding proteins and plasma binding proteins

Question 15

Why doesn’t a change in the concentration of hormone binding / transport protein necessarily result in symptoms of endocrine disease?

Answer 15

This is because it is the free hormone (rather than the bound hormone) that is metabolically active. As long as the hormone feedback loop is in tact, an excess or deficiency state will not result because the free hormone concentration will be maintained within normal limits. A significant change in TOTAL hormone concentration may however be apparent

Question 16

What should current medications be considered when assessing a patient’s hormone concentrations?

Answer 16

Some drugs can displace other substances from protein bindings sites on transport proteins (e.g. albumin)

Question 17

Describe the general mechanism of action of polypeptide hormones (reference image - page 11 of Module 3)

Answer 17

They bind to a specific cell surface receptor which results in either altered membrane permeability to certain substances OR alters the activity of a membrane protein

Many stimulate a change in the membrane protein adenyl cyclase or guanyl cyclase. These proteins catalyse the conversion of ATP and GTP to cyclic AMP or cyclic GMP. This results in information transfer to the intracellular space

Question 18

Describe the general mechanism of action of steroid hormones (reference image - page 11 of Module 3)

Answer 18

1. Pass (easily) through the cell membrane
2. Bind hormone specific receptor proteins in the intracellular space
3. Steroid-receptor complex is transported to the nucleus
4. Within the nucleus the steroid binds to a nuclear acceptor
5. Binding results in modified transcription of particular mRNA
6. Synthesis of cytoplasmic proteins is altered / enhanced
7. These cytoplasmic proteins mediate the effects of the hormone

Question 19

Describe the mechanism of action of amino acid derivatives

Answer 19

Effects are mediated via their release at sympathetic nerve endings. They act via membrane modification (e.g. altered membrane permeability to ions) AND via adenyl cyclase to produce intracellular responses

Question 20

What are the overarching mechanisms by which an endocrinopathy can result?

Answer 20

1. Deficiency of a hormone
2. Excess hormone action
3. Resistance to hormone action

Question 21

What are the general mechanisms by which endocrine hypofunction can result?

Answer 21

Congenital or acquired disorders of:
1. Gland structure
2. Hormone production
3. Hormone action

Question 22

List 7 general reasons why symptoms of endocrine hypofunction can occur

Answer 22

1. Insensitivity of target hormone receptor
2. Non-functional neoplasia (usually pituitary)
3. Inflammatory destruction (usually autoimmune)
4. Congenital defects of gland
5. Defects in metabolic pathway of hormone production or release
6. Idiopathic atrophy
7. Failure of tropic hormone secretion or action

Question 23

Four general types of hormone excess are recognised. What are these?

Answer 23

1. Abnormalities of feedback regulation
2. Exogenous hormone administration
3. Functional neoplasia
4. Production of a hormone like substance

Question 24

List the ways in which hormone function can be evaluated

Answer 24

1. Basal plasma concentrations
2. Urinary excretion
3. Secretion and production rates (require radioactive isotopes - as such, not a common method in vet med)
4. Hormone receptors (in target tissues or fibroblasts) and antibodies (to hormones or target tissues)
5. Tissue / effector organ effects (e.g. water derivation testing)
6. Dynamic tests of hormone reserve / regulation (i.e. stimulation or suppression tests)

Question 25

What are factors that can affect normal basal plasma concentrations of a hormone?

Answer 25

1. Variation due to disease
2. Binding of hormone to transport proteins may affect measurement of total plasma concentration but may not reflect free component
3. Individual variation
4. Some hormones are secreted in response to changes in metabolism (e.g. insulin)

Question 26

What are the limitations in measuring excretion of a hormone in the urine and how can these be mitigated?

Answer 26

Changes in renal function will alter rates of hormone excretion. This can be corrected by measuring urine creatinine (urine cortisol / creatinine ratio)

Values are meaningless if the hormone is not excreted in the urine

Question 27

The diagnosis of many clinical endocrinopathies is based on …

Answer 27

1. Clinical signs
2. Supportive clinical pathology
3. Confirmation of an abnormal hormone concentration
4. Function testing if required

Question 28

Describe the steps that occur at the molecular level in a radioimmunoassay (RIA)

Answer 28

1. Antigen (hormone) is mixed with an antibody specific to the hormone being assessed
2. A labelled antigen (hormone) is added to the mixture. The labelled antigen competes with the unlabeled antigen for binding sites on the antibody until a dynamic equilibrium is reached. The mixture then contains bound and free fractions of labelled and unlabeled antigen
3. Unbound fractions are washed away
4. The amount of bound labeled antigen is measured using radioactivity detection methods
5. The amount of labeled antigen is inversely proportional to the concentration of unlabeled antigen in the sample
6. Concentration of unlabeled antigen in the sample is determined by comparing the measured radioactivity to standard curves / controls

Question 29

Describe the steps that occur at the molecular level in an immunoradiometric (IRMA) assay

Answer 29

1. Monoclonal antibodies specific to the antigen (hormone) are immobilized on a solid support surface (microplate or bead)
2. Sample is added. If antigen is present it binds to the immobilized antibodies
3. Unbound molecules are removed by washing the solid phase (improved specificity / ensures specificity)
4. A secondary labeled antibody specific to the antigen is then added (recognizes a different epitope on the antigen compared to the immobilized antigen)
5. The mixture is incubated to allow the labeled antigen to bind to the antigen, thereby forming a ‘sandwich’ complex
6. Excess labeled antigen is washed away. Only bound antigen antibody complexes are left behind
7. The quantity of retained radioactivity is then measured. The level of radioactivity is directly proportional to the amount of antigen present in the sample
8. Antigen concentration is determined by comparing the measured radioactivity to a standard curve using known concentrations of the antigen

Question 30

Describe the steps that occur at the molecular level in an enzyme-linked immunosorbent assay (ELISA)

Answer 30

1. Monoclonal antibodies specific to the target antigen are immobilized on a solid support (e.g. a microplate well)
2. The sample is added to the well. If antigen is present it binds to the immobilized antibodies
3. Unbound molecules are removed by washing the microplate wells
4. A second antibody specific to the antigen is added. This antibody is conjugated with an enzyme. This antibody recognizes a different epitope on the antigen compared to the immobilized antigen.
5. The mixture is incubated to allow the enzyme conjugated antibody to bind to the antigen
6. Excess detection antibody is washed away; only the bound antibody antigen complexes remain
7. A substrate specific to the enzyme conjugated to the detection antibody is added. This substrate undergoes a reaction catalyzed by the enzyme resulting in the generation of a detectable signal (e.g. a color change or fluorescent signal)
8. The intensity of the signal is then measured using a spectrophotometer or plate reader. The intensity of the signal is directly proportional to the amount if antigen present in the sample
9. The concentration of antigen in the sample is determined by comparing the measured signal to a standard curve generated using known concentrations of the antigen

Question 31

What does sensitivity refer to in the context of a hormone assay?

Answer 31

Sensitivity is the lowest concentration that can be measured accurately

Question 32

What does precision refer to in the context of a hormone assay?

Answer 32

This is the reproducibility of a result

Question 33

Why should precision be taken into consideration when interpreting the result of a hormone assay?

Answer 33

If a result falls near a cut off limit, precision may mean repeated assays could fall within or out of the reference range. This must be considered when using a cut-off value as an absolute means by which a diagnosis is made.

Question 34

What does specificity refer to in the context of a hormone assay?

Answer 34

Specificity relates to how well a particular antibody recognizes a specific hormone and if closely related hormones can compete for binding sites

Question 35

How does cross reactivity impact the specificity of a hormone assay?

Answer 35

This depends. If the cross reacting substance is either unlikely to be present or is only present in minute amounts then the potential for cross reactivity may be irrelevant.

If there is significant cross reactivity, however, then clinical interpretation can be negatively affected.

Question 36

What criteria is required for the general diagnosis of canine hypothyroidism?

Ref: Is it possible to diagnose canine hypothyroidism, Panciera, D. L., 1999

Answer 36

1. Appropriate clinical signs
2. Abnormal thyroid function tests
3. Ultimately an adequate response to thyroid hormone replacement therapy

Ref: Is it possible to diagnose canine hypothyroidism, Panciera, D. L., 1999

Question 37

Which hormones are produced by the adenohypophysis?

Answer 37

TSH, ACTH, gonadotrophins, prolactin, growht hormone, MSH (melanocyte stimulating hormone)

Question 38

Which hormones are produced by the neurohypophysis?

Answer 38

Oxytocin and vasopressin