Laboratory Investigation of Endocrine Disorders

Question 1

1. Explain the hypothalamic pituitary thyroid axis?

Answer 1

• Thyrotrophin releasing hormone (synthesised in the pituitary and released into the hypophyseal system) causes TSH release which controls the synthesis and release of TH.
• TH are synthesised in the thyroid via series of enzyme catalysed reactions, beginning with uptake of iodine into gland.
• T4 main hormone secreted by thyroid, T3 is more biologically active – mostly formed by peripheral enzyme conversion from T4.

Question 2

2. Is TH under positive or negative feedback?

Answer 2

Negative feedback

circulating TH , causes the hypothalamus to release less TRH->TSH->TH

Question 3

3. What are Thyroid Hormones essential for?

Answer 3

normal growth and development; they increase basal metabolic rate (BMR) and affect many metabolic processes.

Question 4

4. How are the effects of thyroid hormone mediated?

Answer 4

•Effects are mediated via activation of nuclear receptor.

Question 5

5. What is the Total and free amount of T4? in the serum?

Answer 5

Total : 60-150 nmol/L

Free : 11-23 pmol/L

Question 6

6. What % of both T4 and T3 are protein bound?

Answer 6

T4= 99.98

T3=99.66

Question 7

7. What is pmol/L

How do you convert between nmol and pmol?

Answer 7

Picomoles per liter (pmol/L)

1 nmol/L= 1000 pmol/L

Question 8

8. What is the Total and free amount of T3? in the serum?

Answer 8

Total = 1.0-2.9 nmol/L
Free = 4-8 pmol/L

Question 9

9. What is the half life of both T4 and T3?

Answer 9

T4= 6-7 Days
T3= 0.5-1 Days

Question 10

10. What is :
    - Euthyroid
    - Hypothyroid
    - Hyperthyroid

Answer 10

Euthyroid = Normal range of T3 and T4

Hypothyroid = Low levels of T3 and T4

Hyperthyroid = High levels of T3 and T4

Question 11

11. What is the difference between primary and secondary hypo/hyper thyroidism

Answer 11

Primary = Dysfunction in the thyroid gland

Secondary = Dysfunction is with pituitary or hypothalamus (tertiary)

Question 12

12. What is Hyperthyroidism ?

Answer 12

Excessive production of thyroid hormones (thyrotoxicosis)

Question 13

13. What are the clinical features of hyperthyroidism?

Answer 13

oGraves: weight loss, heat intolerance, palpitations, goitre (neck swelling), eye changes (Graves).
oThyroid storm in extreme cases: dangerously high HR, BP and body temperature.

Question 14

14. What are some causes of hyperthyroidism?

Answer 14

Graves disease (most common): due to stimulatory TSH-R antibodies
oToxic multinodular goitre: thyroid nodules (don’t require TSH) produce excess thyroid hormones.
oToxic adenoma: single nodule produces the hormone.
oSecondary: excess TSH production (rare)

Question 15

15. What is hypothyroidism?

Answer 15

Deficient production of thyroid hormones

Question 16

16. What are the clinical features of hypothyroidism?

Answer 16

o weight gain, cold intolerance, lack of energy, goitre(swelling of the thyroid gland )
o congenital - developmental abnormalities due to reduced TH from birth.

Question 17

17. How do you know if hypothyroidism is primary or secondary?

Answer 17

oRaised TSH, reduced T4 suggests primary failure.

oReduction in TSH and T4 suggests secondary (hypopituitarism).

Question 18

18. What are some causes of hypothyroidism?

Answer 18

oAutoimmune thyroiditis (Hashimoto’s) - Thyroid peroxidase antibodies (anti-TPO) so less TH produced.
oIodine deficiency e.g. certain mountainous regions.
oToxic adenoma
oSecondary – lack of TSH

Question 19

19. Case study:
A 48 year-old woman presented to her GP with palpitations, weight loss and heat intolerance. A serum sample gave the following results:
Free T4: 31 pmol/L	(11-23)
Free T3 : 12 pmol/L (3-8.8)
TSH     : 0.08 mU/L (0.5-4.0)
Diagnosis?

Answer 19

-Hyperthyroidism

Question 20

20. Case Study:
A 70 year old man complained of tiredness and weight gain. His heart rate was 55 beats/min. Thyroid function tests were carried out: 
Free T4 :10 pmol/L	(11-23)
TSH	 : 18.4 mU/L	(0.5-4.0)
Diagnosis?

Answer 20

Primary hypothyroidism

Question 21

21. Blood flows from the outer adrenal cortex to where?

Answer 21

•Blood flows from outer cortex (steroids e.g. mineralocorticoids, glucocorticoids, and androgen) to inner medulla (adrenaline and noradrenaline).

Question 22

22. What impact do layer-specific enzymes have on steroid synthesis?

Answer 22

•Layer-specific enzymes mean steroid synthesis in one layer can inhibit different enzymes in subsequent layers–> functional zonation of cortex (diff hormones made in each layer)

Question 23

23. What hormones are made in the :
    - Zona Glomerulosa
    - Zona Fasciculata
    - Zona Reticularis

Answer 23

Zona Glomerulosa = Mineralocorticoids
Zona Fasciculata = Glucocorticoids
Zona Reticularis = Androgens

Question 24

24. How are steroids of the adrenal cortex transferred from outside to inside ?

Answer 24

Via Capillary Network

Question 25

25. Fill in the blank: | "Enzymatic modification of ********** allows the production of different steroids"

Answer 25

Cholesterol

Question 26

26. Following questions are about Mineralocorticoids: - Give an example of one - What are they involved in - What is the synthesis and release regulated by?

Answer 26

e.g. aldosterone salt and water balance in order to maintain plasma volume by sodium retention: maintenance of blood pressure over the long term (by maintaining blood volume). oSynthesis and release regulated by hypothalamic-pituitary-adrenal axis (CRH, ACTH)

Question 27

27. Following questions are about Glucocorticoids: - Give an example of one - What are they involved in - What is the synthesis and release regulated by?

Answer 27

e.g. cortisol involved in metabolism and immune function and controlled by RAAS. oStress increases release, but minimal levels essential for normal function.

Question 28

28. What are androgens secreted in response to?

Answer 28

ACTH

Question 29

29. What are the main effects cortisol has on the CVS?

Answer 29

maintains blood pressure by counter-balancing NO

Question 30

30. What effect does cortisol have on insulin resistance?

Answer 30

promotes insulin resistance in skeletal muscle to preserve glucose for other essential tissues e.g. brain, promotes gluconeogenesis and lipolysis etc.

Question 31

31. What causes insulin secretion and lipogenesis?

Answer 31

Hyperglycaemia

Question 32

32. What type of receptor is ACTH?

Answer 32

G-protein couples

Question 33

33. What does the ACTH receptor stimulate the uptake of ?

Answer 33

via cAMP stimulates cholesterol uptake and steroid synthesis

Question 34

34. What syndrome is associated with: - Aldosterone Excess - Cortisol Excess

Answer 34

* Aldosterone excess: Conn’s syndrome. | * Cortisol excess: Cushing’s syndrome.

Question 35

35. What is the synthesis and release of: Cortisol Aldosterone Adrenal Androgens regulated by?

Answer 35

Cortisol: synthesis and release regulated by hypothalamic-pituitary-adrenal axis (CRH, ACTH) Aldosterone: controlled by RAAS Adrenal androgens: ACTH (not gonadotropins)

Question 36

36. Explain the feedback system for cortisol?

Answer 36

•Exogenous glucocorticoid negatively feedback to the hypothalamus and pituitary which reduces cortisol production.

Question 37

37. How is the feedback loop of cortisol disturbed in cushings syndrome (`excess cortisol)

Answer 37

•Raised ACTH from pituitary adenoma causes raised increased cortisol production which feedbacks to the hypothalamus and anterior pituitary to reduce CRH but ACTH remains being produced---> increased cortisol

Question 38

38. What is Dexamethasone?

Answer 38

Dexamethasone is a steroid that prevents the release of substances in the body that cause inflammation

Question 39

39. What is A Dexamethasone (exogenous synthetic glucocorticosteroids) Suppression Test - how can we interpret the results

Answer 39

•Low doses will normally suppress ACTH secretion via negative feedback. •LOW dose fails to suppress ACTH secretion with pituitary disease (Cushing's) as it’s not enough for the new higher negative feedback threshold, but .Higher dose will suppress ACTH secretion in Cushing's. •No suppression with low or high dose: suggests ectopic source of ACTH (e.g. secondary tumour).

Question 40

40. If you conduct a LOW dose dexamethasone suppression test , what result (ie suppression or not) would you expect in : - Cushing's disease - Adrenal Tumour - Ectopic ACTH

Answer 40

- Cushing's disease = No suppression - Adrenal Tumour= No suppression - Ectopic ACTH= No suppression

Question 41

41. If you conduct a HIGH dose dexamethasone suppression test , what result (ie suppression or not) would you expect in : - Cushing's disease - Adrenal Tumour - Ectopic ACTH

Answer 41

Cushing’s disease=Suppression Adrenal tumour = No suppression Ectopic ACTH =No suppression

Question 42

42. What level of plasma ACTH would you expect in : - Cushing's disease - Adrenal Tumour - Ectopic ACTH

Answer 42

- Cushing's disease = High - Adrenal Tumour = Low - Ectopic ACTH = Very high

Question 43

43. What is Addison's disease?

Answer 43

* Primary adrenocortical failure - usually autoimmune | * Loss of cortisol, androgens and aldosterone and High ACTH.

Question 44

44. What is secondary adrenocortical insufficiency ?

Answer 44

impaired ACTH release: Head trauma, tumour, surgery, Abrupt steroid withdrawal.

Question 45

45. What are we testing for in adrenal function tests?

Answer 45

•Assess ability of adrenal to produce cortisol in response to ACTH.

Question 46

46. How does the short synacthen (synthetic ACTH) test work?

Answer 46

oMeasure baseline cortisol (9am) and 30 min after 250 µg synacthen (synthetic ACTH) i.m. o Adrenal insufficiency is excluded by an increase in cortisol of >200 nmol/L and/or a 30 min value >550

Question 47

47. How does the long synacthen (synthetic ACTH) test work?

Answer 47

o Adrenal cortex ‘shuts down’ in absence of stimulation by ACTH – time needed to regain responsiveness o 3-day stimulation with i.m. synacthen o In secondary (but not primary) adrenal insufficiency cortisol increases by >200 nmol/L over baseline o Long test not often necessary since ACTH assay can distinguish.

Question 48

48. o A 51 year old woman was admitted to hospital with a history of weight loss and lethargy. She reported that she felt dizzy when she stood up. o Biochemical tests revealed that she was hyponatraemia and she was given i.v. saline. o A synacthen test was carried out with the following results: o Baseline cortisol 342 nmol/L (ref. range150-650) o 30 min post synacthen. 419 nmol/L o How would you interpret these results? What further tests should be carried out?

Answer 48

Her cortisol levels should have increased by 200 or at least been 500 in total It wasn't This suggests the adrenal cant produce cortisol in response to ACTH -> Should take the long synacthen test to confirm adrenal insufficiency