Thyroid Function Lab Testing

Question 1

What are the three plasma proteins that transport thyroid hormones?

Answer 1

TBG- Thyroxine Binding Globulin. TBPA-Thyroxine binding pre-albumin. Albumin

Question 2

What is the best assessment of thyroid function?

Answer 2

TSH assuming steady state conditions and the absence of pituitary or hypothalamic disease.
third generation assay is the MOST sensitive

Question 3

What are the thyroid antibody tests and which diseases do they correlate with?

Answer 3

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

Question 4

What happens to TSH as you age?

Answer 4

shifts towards higher TSH in older patients, usually in their 7th decade.

Question 5

What conditions increase total T4 levels?

Answer 5

Hyperthyroidism, Acute thyroiditis, Conditions causing increased TBG , Pregnancy

Question 6

What conditions decrease total T4 levels?

Answer 6

Hypothyroid states, Pituitary insufficiency, Hypothalamic failure, Protein malnutrition/depletion, Iodine insufficiency, Cushings, cirrhosis, advanced cancer

Question 7

Describe the advantages and disadvantages of a free T4 test?

Answer 7

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.

Question 8

What is the potential cause behind normal thyroid function but unexplained high or low T4/T3?

Answer 8

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.

Question 9

What conditions increase TBG levels?

Answer 9

pregnancy, infectious hepatitis, high estrogen levels

Question 10

What conditions decrease TBG levels?

Answer 10

malnutrition, stress, steroids, high testosterone levels, phenytoin, propanolol, menopause

Question 11

Describe the use of free T4 index and how it’s calcuated

Answer 11

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

Question 12

What does the T3 resin uptake measure?

Answer 12

unoccupied binding sites on TBG. It’s not a measure of T3

Question 13

How are total T3 results interpreted?

Answer 13

Increased in hyperthyroidism, pregnancy, conditions that increase proteins, estrogen, Oral contraceptives. Decreased by: androgens, phenytoin, propranolol, high-dose salicylates.

Question 14

What do anitthyroid peroxidase (TPO Ab) and antithyroglobulin (Tg Ab) antibodies attack?

Answer 14

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

Question 15

What are indications to test for antithyroid peroxidase (TPO Ab) and antithyroglobulin (Tg Ab) antibodies?

Answer 15

Hashimoto’s thyroiditis, Grave’s Disease, myxedema

Question 16

What are normal thyroid autoantibody levels and what conditions can affect test results?

Answer 16

Normal titers: <1:100. Rheumatic autoimmune diseases (including RA), pernicious anemia, thyroid carcinoma

Question 17

What is thyroglobulin (Tg)?

Answer 17

Protein precursor of thyroid hormones. Levels are low or undetectable with normal thyroid function. Used as a tumor marker of thyroid tissue

Question 18

What is the first test for evaluating thyroid nodules?

Answer 18

Measure TSH. If it is low usually probable overt hyperthyroidism. If it is normal or high more suspicious for cancer

Question 19

Why is a thyroid ultrasound of nodules helpful after measuring the TSH?

Answer 19

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

Question 20

What ultrasound characteristics suggest a benign thyroid nodule?

Answer 20

Well demarcated. Fluid filled (cystic), Multiple nodules, no blood supply – not live tissue

Question 21

What is the most accurate method for evaluating thyroid nodules?

Answer 21

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

Question 22

How are thyroid nodules managed after US assessment?

Answer 22

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

Question 23

Why is a radionuclide thyroid scan done?

Answer 23

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

Question 24

What is the difference between a hot and cold nodule?

Answer 24

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

Question 25

What are CI to a thyroid scan?

Answer 25

pregnant/breastfeeding, allergies (iodine, shellfish, bee venom), meds (thyroid hormones, antithyroid meds, iodine containing meds- cough syrups, multivitamins, amiodarone)

Question 26

What are the benefits of a fine needle aspiration?

Answer 26

Non-surgical differentiation of malignant and benign nodules. Cost effective, safe

Question 27

What do benign nodules consist of?

Answer 27

follicular epithelium with variable amount of colloid

Question 28

What thyroid cancers cannot be diagnosed with a fine needle aspiration?

Answer 28

Follicular Ca and Hurthle cell CA