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Chemical Pathology > Thyroid Function Tests > Flashcards

Thyroid Function Tests Flashcards

1
Q

What are the steps in the formation and release of thyroid hormones

A

Trapping of iodide
●Oxidation of iodide to iodine
●Incorporation of iodine into tyrosyl residues
●Coupling of iodo-tyrosyl residues
●Release of T3 and T4 from thyroglobulin

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2
Q

What is dyshormonogenesis

A

“Blocks” or difficulty in making thyroid hormones

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3
Q

What are some enzymes used in thyroid formation

A

Trapping enzyme
Peroxidase
Iodinase
Coupling enzyme

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4
Q

Give some examples of substances that inhibit iodinases

A

Carbimazole
Propylthiouracil
Sulphonamides

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5
Q

Give some examples of substances that inhibit peroxidases

A

Thiourea
Methimazole
Carbimazole

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6
Q

Give an example of substances that inhibit coupling enzyme

A

Thiouracil

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7
Q

Give an example of substances that inhibit trapping enzyme

A

Perchlorate
Thiocyanate
Pertechnetate

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8
Q

What is endemic goitre

A

Iodine deficiency⇒ Low T3 and low T4 ⇒ High TSH
●TSH stimulates all physiological processes of the thyroid gland ⇒ hyperplasia / hypertrophy of the thyroid

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9
Q

What is Sick euthyroid syndrome:
(Euthyroid sick syndrome)

A

In the very old, starving, severely ill, acutely ill and after trauma (e.g. surgery) and drugs: (propranolol, amiodarone, radio contrast media), there is:
●Increased metabolism of T4 to RT3
●Plasma T3 level falls
●There is minimization of BMR
NB: 20% of T3 is made by the thyroid gland and 80% is made by mono-deiodination of T4

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10
Q

How are thyroid hormones transported

A

They are transported as protein-bound and free hormone in equilibrium
●99.95% T4 and 99.5% T3 is protein bound
●Protein binding is so strong that only 0.05% of T4 and 0.5% T3 is free hormone

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11
Q

What percentage of T3 and T4 are bound to Thyroxine binding pre-albumin (TBPA, Transthyretin)

A

15

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12
Q

What percentage of T3 and T4 are bound to Albumin (Alb)

A

10

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13
Q

What percentage of T3 and T4 are bound in Thyroxine binding globulin (TBG)

A

75

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14
Q

What is hereditary TBG excess

A

Causes an increase in TT4 and TT3 levels
●Benign
●Needs to be recognized to prevent unnecessary treatment

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15
Q

What are some factors that increase TBPs

A

Pregnancy (TBPs are 30-40% higher than in the non-pregnant)
●Oestrogens (including high levels in the newborn)
●Oestrogen containing contraceptives
●Hereditary TBG excess

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16
Q

What are some factors that decrease the TBPs leading to low TT4 levels

A

Chronic liver disease
●Nephrotic syndrome
●Malnutrition
●Drugs that displace T3 and T4 from TBPs
●Intake of androgens or danazol
●Severe or chronic illness especially in the elderly
●Inherited TBG deficiency (rare)

17
Q

What are some other causes of low TT4

A

Increased peripheral metabolism of T4
●Androgens
●Corticosteroids
●Some anticonvulsants (e.g. phenytoin)

Inhibition of secretion of thyroid hormones
●Lithium
●Phenylbutazone

18
Q

What is the importance of free T3 and T4

A

They are diffusible
●They are responsible for the metabolic effects of thyroid hormones
●They regulate the output of TSH
●They are kept fairly constant by homeostatic mechanisms

19
Q

Give some examples of thyroid function tests

A

Total thyroxine, TT4
●Total tri-iodothyronine, TT3
●Free thyroxine, FT4
●Free tri-iodothyronine, FT3

(Instant)Thyroid stimulating hormone, TSH
●Tests of H-P-Th axis, TSH and TRH dynamic tests
●In vivo radio-active uptake tests using 131I 132I and 99Tc
●Thyroid auto-antibodies, TSI, LATS, LATSP, colloid antibodies

20
Q

What is total thyroxine

A

Changes in the levels of TBPs can cause misleadingly high or low results
●TBP-T4 ⇋ FT4 + U-TBP
●>99.9% of T4 is present as TBP-T4
●Therefore TT4 effectively measures TBP-T4

21
Q

What is total tri-iodothyronine (TT3)

A

There is little cross-reaction with T4
●Main use is in the diagnosis of hyperthyroidism
●In most cases of thyrotoxicosis TT4, FT4, TT3 , FT3 
● In a few cases of thyrotoxicosis TT4, FT4 , TT3 , FT3  (T3 toxicosis)
Total tri-iodothyronine (TT3) is usually  in hypothyroidism
●It is less sensitive for hypothyroidism than plasma TT4
●Hypoactive thyroid glands produce an  proportion of T3
●There are  levels of T3 in the old, severely ill, after trauma and in certain acute illnesses (e.g. MI)

22
Q

What are free thyroid hormones (FT3, FT4)

A

More accurate, (i.e. sensitive and specific) tests of thyroid status than TT3, and TT4
Measured by:
●Equilibrium dialysis with RIA, EMIT, IMA
Estimated by:
●Free thyroxine index (FTI)
●Total T4 : TBG ratio
●Because FT4 is now readily measured, only a few laboratories still determine FTI and TT4: TBG ratio

23
Q

What is TSH

A

Some methods cannot distinguish between subnormal and low normal TSH levels (cannot be used in diagnosing hyperthyroidism)
●Valuable measurement in early 1 hypothyroidism: Plasma T3 (F/T),T4 (F/T)  and in-vivo radio-active uptake tests are normal but TSH 
●Also important in 2 hypothyroidism: TSH 

24
Q

What are In vivo radio-active uptake tests

A

Measure the uptake of an oral dose of radioactive iodine (131I; half-life 8 days 132I; half-life 140 min)
●I.V. Technetium 99m (99mTc; half-life, 6h)

25
Q

What are the uses of an in vivo radio-active uptake test

A

●Help with calculation of dose of radio-activity required in treatment of hyperthyroidism (99mTc unsuitable)
●Monitor thyroid function in patients treated with anti-thyroid drugs
●With scanning of the thyroid to determine if a nodule is “hot” or “cold”
●Determine if there is extra-thyroidal functioning thyroid tissue

26
Q

What are thyroid antibodies

A

Complement-fixing antibodies specific for thyroid tissue
Present in >80% of patients with
Hashimoto’s disease
(anti-TPO, anti-Tg, TSH receptor blocking antibodies)

Antibodies to thyroglobulin:
●Can be detected in most cases of early or incipient hypothyroidism
●80% of hyperthyroid patients also have antibodies to thyroglobulin in the serum
●Thyroglobulin antibodies found in a small proportion of healthy individuals
●Thyroid microsomal antibodies are found in Grave’s and Hashimoto’s diseases
●Antibodies to a second colloid antigen is reported in all forms of auto-immune thyroiditis and in de Quervain’s thyroiditis

Thyroid stimulating immunoglobulins (TSI formerly known as LATS)
●In Grave’s disease IgG antibodies directed against TSH receptors are present: binding to receptors leading to thyroxine production
●Others; thyroid growth immunoglobulins (TGI) stimulate thyroid growth but not hormone production

27
Q

What are some miscellaneous tests to take

A

●Basal metabolic rate (BMR)
●Glucose tolerance tests
●Plasma calcium
●Plasma LDL-cholesterol
●Plasma creatine kinase

28
Q

What is the reference interval for FT3

A

3.0-8.6 pmol/L

29
Q

What is the reference interval for FT4

A

9-23 pmol/L

30
Q

What is the reference interval for TT4

A

55-140 nmol/L

31
Q

What is the reference range for TSH

A

0.2-5.5 mU/L

32
Q

What are the reference intervals for TT3

A

1.2-3.4 nmol/L

33
Q

What is the thyroid-binding globulin

A

7-17 mg/L

Chemical Pathology (16 decks)

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