LECTURE - Thyroid Function Testing

Question 1

T or F. T4 is more potent than T3

Answer 1

F! T3 is 3-5x more potent than T4

also provides negative feedback!

Question 2

transport of thyroid hormones in the body

Answer 2

• inactive (protein-bound) is majority
  > bound by TBG (thyroxine-binding globulin (T4 + T3)
• thyroxine-binding pre-albumin (T4)
• albumin (T4 and T3)
• active (Free) <1%

Question 3

euthyroid

Answer 3

thyroid balance for optimal function

Question 4

this is a direct ESTIMATE of FT4

Answer 4

serum-free T4

• reliably measures FT4 immunoassay
• BUT FT4 affected by extreme variations in [TBG]

Question 5

storage protein on which T4 and T3 are assembled in the thyroid

Answer 5

thyroglobulin

Question 6

thyroglobulin is increased in these cases…

Answer 6

malignant, benign, and inflammatory thyroid conditions

small amounts secreted into circulation in parallel with T4 and T3

Question 7

sensitive and specific marker of recurrence of papillary/follicular thyroid cancer following thyroidectomy

Answer 7

thyroglobulin

Question 8

TPO antibodies

Answer 8

• thyroid peroxidase
• key enzyme in thyroid hormone synthesis
• present in Hashimoto’s thyroiditis, Graves’ disease, painless (silent thyroiditis), post-partum thyroiditis subacute (viral) thyroiditis
• changing titre not clinically significant; to be used once-only diagnostic test

Question 9

this controls the basal metabolic rate (calories required to sustain life0

Answer 9

thyroid

- engine of the body

Question 10

T or F. thyroid hormone act on nearly every cell in the body

Answer 10

T!

• regulates breathing, body temp
• affect protein, carbohydrate, and lipid metabolism

Question 11

this is required for neural growth and normal development (esp. brain development and maturation)

Answer 11

thyroid hormones

Question 12

thyroid hormone has a synergy with this other hormone

Answer 12

growth hormone

- regulate long bone growth and neural maturation

Question 13

thyroid hormones increase the body’s sensitivity to _______ (via adrenergic receptors)

Answer 13

catecholamines

- heart rate, muscle strength

Question 14

thyroid hormones also increase ________ and _______ metabolism

Answer 14

calcium and phosphate

Question 15

peripheral conversion of T4 to T3 by deiodinases

Answer 15

occur in the liver, brown fat, kidney

Question 16

rT3

Answer 16

reverse T3 is biologically inactive

Question 17

increased binding to TBG (5)

Answer 17

• estrogen
• pregnancy
• birth control pill
• acute hepatitis
• congenital disorders

Question 18

decreased binding to TBG (5)

Answer 18

• drugs
• corticosteroids
• androgens
• major illness (cirrhosis or cancer for ex)
• congenital

Question 19

where T3 levels are particularly important

Answer 19

T3 toxicosis => hyperthyroidism

Question 20

serum FT3

Answer 20

‘direct’ estimate of free T3

- reliably measure by immunoassay

Question 21

why do we never measure FT3 for hypothyroidism?

Answer 21

hypothyroidism is due to large decrease in hormone

so free T3 wouldn’t really show up … only measure when trying to look for excess

Question 22

which would be the best test for an inactive thyroid?

Answer 22

high TSH

Question 23

this is the test we use to initially assess thyroid function

Answer 23

TSH assay! sensitive!

• VERY commonly ordere
• can separate hyperthyroid and hypothyroid from euthyroid subjects

Question 24

when TSH values might not be reliable (5)

Answer 24

• presence of pituitary/hypothalamic disease
• severe non-thyroidal illness (Euthyroid Sick Syndrome)
• severe psychiatric disease
• some drugs (high corticosteroids, dopamine which suppresses TSH, dopamine agonists)
• after treatment f thyrotoxicosis w radioactive iodine to reduce T3
  > hypothal/pit lose sensitivity to low T3 levels

Question 24

when TSH values might not be reliable

Answer 24

- presence of pituitary/hypothalamic disease - severe non-thyroidal illness (Euthyroid Sick Syndrome) - severe psychiatric disease - some drugs (high corticosteroids, dopamine which suppresses TSH, dopamine agonists) -

Question 25

euthryoid sick syndrome

Answer 25

- ppl in ICU; lots of things affecting their general health - to avoid thyroid probloe => preferential conversion of T4 to rT3 (INACTIVE) - presumed physiological adaptation to severe physical stress - patients do not benefit from L-T4 replacement

Question 26

inflammation of thyroid gland

Answer 26

Hashimoto's thyroiditis

Question 27

these can be increased in thyroid inflammation or thyroiditis

Answer 27

TPO Abs | thyrodi peroxidase Abs

Question 28

antibody responsible for hyperthyroidism in Graves' disease

Answer 28

TSH receptor Ab

Question 29

TSH receptor antiuobdy

Answer 29

- Graves' disease - binds to TSH receptor and activates receptor resulting in hyperthyroidism - indirectly measured! measure ability of patient immunoglobulin to inhibit binding of radiolabeled TSH to its receptor - does not measure stimulatory activity

Question 30

TSH receptor antibodies can also present as blocking antibodies

Answer 30

- Hashimoto's thyroiditis | - antibodies have inhibitory effect (opposite to Graves')

Question 31

TSH receptor anibody test is gold standard for iagnosis of this disease

Answer 31

Graves' disease in a hyperthyroid subject

Question 32

this is a partiucular concern in a mom who has Graves' disease (or history of)

Answer 32

neonatal hyperthyriodism | - useful prediction test =- TSH receptor Ab test