Flashcards in Thyroid Function Lab Testing Deck (28):
What are the three plasma proteins that transport thyroid hormones?
TBG- Thyroxine Binding Globulin. TBPA-Thyroxine binding pre-albumin. Albumin
What is the best assessment of thyroid function?
TSH assuming steady state conditions and the absence of pituitary or hypothalamic disease.
third generation assay is the MOST sensitive
What are the thyroid antibody tests and which diseases do they correlate with?
TPO-Ab and Tg-Ab (high concentrations are seen in nearly all patient’s with Hashimoto's thyroiditis). TSH receptor stimulating Ab seen in Grave's disease. TSH receptor blocking Ab seen in atrophic Hashimoto's thyroiditis & sometimes in Grave's
What happens to TSH as you age?
shifts towards higher TSH in older patients, usually in their 7th decade.
What conditions increase total T4 levels?
Hyperthyroidism, Acute thyroiditis, Conditions causing increased TBG , Pregnancy
What conditions decrease total T4 levels?
Hypothyroid states, Pituitary insufficiency, Hypothalamic failure, Protein malnutrition/depletion, Iodine insufficiency, Cushings, cirrhosis, advanced cancer
Describe the advantages and disadvantages of a free T4 test?
More accurate. Gives a quicker result to response to therapy with replacement thyroxine then TSH. Fewer interfering factors. increased by: heparin, ASA, propranolol. Decreased by: furosemide, phenytoins. Cannot be measure directly.
What is the potential cause behind normal thyroid function but unexplained high or low T4/T3?
either an increase or decrease of TBG. If there is an excess or deficiency of this protein it alters the T4 or T3 measurement but does not affect the action of the hormone.
What conditions increase TBG levels?
pregnancy, infectious hepatitis, high estrogen levels
What conditions decrease TBG levels?
malnutrition, stress, steroids, high testosterone levels, phenytoin, propanolol, menopause
Describe the use of free T4 index and how it's calcuated
indirectly measures unbound T4 and corrects misleading results of total T4 caused by conditions that alter TBG. Calculated product of T3 resin uptake and serum T4
What does the T3 resin uptake measure?
unoccupied binding sites on TBG. It's not a measure of T3
How are total T3 results interpreted?
Increased in hyperthyroidism, pregnancy, conditions that increase proteins, estrogen, Oral contraceptives. Decreased by: androgens, phenytoin, propranolol, high-dose salicylates.
What do anitthyroid peroxidase (TPO Ab) and antithyroglobulin (Tg Ab) antibodies attack?
work against thyroid peroxidase, an enzyme that plays a part in the T4-to-T3 conversion and synthesis process. Causes chronic inflammation and chronic thyroiditis. leads to hypothyroid
What are indications to test for antithyroid peroxidase (TPO Ab) and antithyroglobulin (Tg Ab) antibodies?
Hashimoto's thyroiditis, Grave's Disease, myxedema
What are normal thyroid autoantibody levels and what conditions can affect test results?
Normal titers: <1:100. Rheumatic autoimmune diseases (including RA), pernicious anemia, thyroid carcinoma
What is thyroglobulin (Tg)?
Protein precursor of thyroid hormones. Levels are low or undetectable with normal thyroid function. Used as a tumor marker of thyroid tissue
What is the first test for evaluating thyroid nodules?
Measure TSH. If it is low usually probable overt hyperthyroidism. If it is normal or high more suspicious for cancer
Why is a thyroid ultrasound of nodules helpful after measuring the TSH?
differentiates cystic from solid nodules, can aid in determining which nodule(s) to biopsy, allows measurement of a nodule's size and if a nodule is getting smaller or larger. Aids in thyroid fine needle biopsy. will not tell if a nodule is benign or malignant
What ultrasound characteristics suggest a benign thyroid nodule?
Well demarcated. Fluid filled (cystic), Multiple nodules, no blood supply – not live tissue
What is the most accurate method for evaluating thyroid nodules?
fine needle aspiration biopsy. done for nodules > 1 – 1.5 cm w/ suspicious findings on US. If there are risk factors nodules =/> 0.5 cm are biopsied
How are thyroid nodules managed after US assessment?
If serum TSH is normal or elevated then FNA biopsy is indicated. If serum TSH is LOW it can indicate hyperthyroidism or hyperfunctioning nodule and a radioactive thyroid scan needs to be done
Why is a radionuclide thyroid scan done?
determine functionality of nodule, measure size of goiter, follow-up w/thyroid cancer pts after surgery, identifies hot or cold nodules, locates thyroid tissue outside the neck, and used to select nodules for FNA
What is the difference between a hot and cold nodule?
Hot nodule – rapid uptake of iodine or isotope. Less likely to be malignant. Cold nodule – little or no uptake of iodine or isotope. More likely to be malignant
What are CI to a thyroid scan?
pregnant/breastfeeding, allergies (iodine, shellfish, bee venom), meds (thyroid hormones, antithyroid meds, iodine containing meds- cough syrups, multivitamins, amiodarone)
What are the benefits of a fine needle aspiration?
Non-surgical differentiation of malignant and benign nodules. Cost effective, safe
What do benign nodules consist of?
follicular epithelium with variable amount of colloid