LECTURE - Thyroid Function Testing Flashcards
T or F. T4 is more potent than T3
F! T3 is 3-5x more potent than T4
also provides negative feedback!
transport of thyroid hormones in the body
- 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%
euthyroid
thyroid balance for optimal function
this is a direct ESTIMATE of FT4
serum-free T4
- reliably measures FT4 immunoassay
- BUT FT4 affected by extreme variations in [TBG]
storage protein on which T4 and T3 are assembled in the thyroid
thyroglobulin
thyroglobulin is increased in these cases…
malignant, benign, and inflammatory thyroid conditions
small amounts secreted into circulation in parallel with T4 and T3
sensitive and specific marker of recurrence of papillary/follicular thyroid cancer following thyroidectomy
thyroglobulin
TPO antibodies
- 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
this controls the basal metabolic rate (calories required to sustain life0
thyroid
- engine of the body
T or F. thyroid hormone act on nearly every cell in the body
T!
- regulates breathing, body temp
- affect protein, carbohydrate, and lipid metabolism
this is required for neural growth and normal development (esp. brain development and maturation)
thyroid hormones
thyroid hormone has a synergy with this other hormone
growth hormone
- regulate long bone growth and neural maturation
thyroid hormones increase the body’s sensitivity to _______ (via adrenergic receptors)
catecholamines
- heart rate, muscle strength
thyroid hormones also increase ________ and _______ metabolism
calcium and phosphate
peripheral conversion of T4 to T3 by deiodinases
occur in the liver, brown fat, kidney
rT3
reverse T3 is biologically inactive
increased binding to TBG (5)
- estrogen
- pregnancy
- birth control pill
- acute hepatitis
- congenital disorders
decreased binding to TBG (5)
- drugs
- corticosteroids
- androgens
- major illness (cirrhosis or cancer for ex)
- congenital
where T3 levels are particularly important
T3 toxicosis => hyperthyroidism
serum FT3
‘direct’ estimate of free T3
- reliably measure by immunoassay
why do we never measure FT3 for hypothyroidism?
hypothyroidism is due to large decrease in hormone
so free T3 wouldn’t really show up … only measure when trying to look for excess
which would be the best test for an inactive thyroid?
high TSH
this is the test we use to initially assess thyroid function
TSH assay! sensitive!
- VERY commonly ordere
- can separate hyperthyroid and hypothyroid from euthyroid subjects
when TSH values might not be reliable (5)
- 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
when TSH values might not be reliable
- 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)
euthryoid sick syndrome
- 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
inflammation of thyroid gland
Hashimoto’s thyroiditis
these can be increased in thyroid inflammation or thyroiditis
TPO Abs
thyrodi peroxidase Abs
antibody responsible for hyperthyroidism in Graves’ disease
TSH receptor Ab
TSH receptor antiuobdy
- 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
TSH receptor antibodies can also present as blocking antibodies
- Hashimoto’s thyroiditis
- antibodies have inhibitory effect (opposite to Graves’)
TSH receptor anibody test is gold standard for iagnosis of this disease
Graves’ disease in a hyperthyroid subject
this is a partiucular concern in a mom who has Graves’ disease (or history of)
neonatal hyperthyriodism
- useful prediction test =- TSH receptor Ab test
