Laboratory test most useful in screening for thyroid dysfunction
TSH (thyrotropin assay), 3rd generation
What is the pathway to thyroid hormone synthesis & secretion?
Hypothalamus releases TRH (thyrotropin- releasing hormone) to the anterior pituitary which releases TSH (thyroid stimulating hormone/thyrotropin) which attaches to receptors on thyroid gland in response to low circulating thyroid hormone (T4/T3) - Negative feedback loop.
What is thyroid peroxidase?
An enzyme in the thyroid gland that oxidizes iodide to iodine to be used in the making of T4/T3.
Which of the thyroid hormones is more potent and therefore more biologically active?
T3 - 3-5x more potent than T4. Most of this hormone is actually from the peripheral tissue conversion of T4 to T3. Only 20% is actually synthesized in the thyroid gland.
T or F : Thyroid hormones are mostly in bound form to carrier proteins such as TBG (thyroxine binding globulin), thyroxine-binding prealbumin (TBPA, transthyretin) and albumin.
TRUE, All but 0.03% of T4 and 0.3% of T3 is protein bound. The free concentration is responsible for biologic effects of the thyroid hormones.
Other laboratory methods in thyroid testing...
Measurement of thyroid gland autoantibodies (anti-thyroid peroxidase (TPOab), antithyroglobulin), and antibodies against the thyrotropin receptor.
When an abnormal TSH is received, what is the next test?
free T4 level - shows an inverse correlation to the serum thyrotropin level.
With a normal TSH, but abnl serum FT4 result, what can be the cause?
abnormality in protein binding of thyroid hormone.
What is a better way to monitor treatment of hyperthyroidism, at least in the beginning?
FT4 - level normalizes much sooner and is a better marker to gauge effectiveness of the therapy in the first few months of treatment. TSH levels may take many months to respond.
Normal adult thyrotropin levels
Normal Total T4 (adult)
Normal Free T4 (adult)
Normal Total T3 (adult)
Normal Free T3 (adult)
elevated TSH, low T4
More prevalent in women
Caused by loss of functional thyroid tissue from infiltrative diseases to thyroid (bacterial, viral), autoimmune thyroiditis, thyroid sx, radiation injury to neck, defects in synthesis caused by iodine deficiency, antithyroid drugs, medications (steroids), congenital defects of enzymes.
causes outside of thyroid gland itself
- less common
- deficiency in pituitary hormone secretion or hypothalamic disease