LABORATORY EVALUATION OF THYROID FUNCTION

Note--TFT abnormalities may not portend functional deficits
  1. In a prospective study of a population of pts > 85yo, 4y incidence of disability, depression, and cognitive impairment was not ass'd with baseline TSH or free T4 levels (JAMA 292:2591, 2004--JW)
  2. Magnitude of TSH increase correlated poorly with clinical manifestations of hypothyroidism in one case series (BMJ 326:295, 2003--JW)
I. Specific Tests
  1. TSH
    1. The main test used to Screen for abnormalities of thyroid hormone production and to monitor treatment
  1. T3 and T4
  1. Usually T4 is the one that's measured, because it's the predominant one in the circulation
  2. Free T4 index--Better than (total) serum T4: total serum T4 changes with thyroid-binding globulin & other serum binding proteins, but free T4 doesn't change
  3. T3 or Free T3 measurements have no purpose--nl in 20-30% of hypothyroid pts; low in about 70% of hosp'd pts with no thyroid disease
  1. I-131 uptake
  1. Determination of the proportion of extrathyroidal iodine taken up by thyroid in a 24h period
  2. Give a dose of I-131 and measure radioactivity of thyroid 24h later
  3. Using estimation of extracellular volume, can determine proportion of total-body-iodine taken up by thyroid in 24h (nl = 10-35%)
  4. Presumably this gives a measure of how active the thyroid is, because a thyroid busy taking up iodide should be busy making T3 and T4
  1. Antithyroid microsomal Ab titers--elevated in Hashimoto's and Postpartum thyroiditis
  2. Thyroid suppression test (notes from 1990; may have become obsolete with newer high-sensitivity assays for TSH
  1. Measure I-131 uptake before and after a large dose of thyroid hormone
  2. If thyroid is functioning normally, will see a decrease in I-131 uptake because of decreased TSH production. If no decrease, then thyroid is functioning autonomously
  1. TRH stimulation test (notes from 1990; may have become obsolete with newer high-sensitivity assays for TSH)
  1. To test anterior pituitary function
  2. Give TRH, measure change in TSH; should get a rise
II. Patterns of TFT abnormality
  1. Hypothyroidism due to low TSH us. accompanied by other deficiencies of pituitary hormones
  2. Low Free T4 & nl TSH: think abnl TSH with reduced biological activity
  3. Free T4 is high in 95% of hyperthyroid pts; 5% have isolated T3-thyrotoxicosis (measure with Free T3)
  4. Low TSH with nl Free T3 & Free T4: think either meds that suppress TSH secretion (see below) or poss. mildly thyrotoxic with min. hypersecretion of T4. F/U with T3-suppression test or test for TRH stim. of TSH release
  5. High Free T4 but nl TSH: may have circulating anti-T4 Ab's or familial dysalbuminemic hyperthyroxinemia
  6. Free T4 and TSH should be normal in pregnancy
III. Conditions which alter results of TFT's:
  1. Illness--us. temporary alterations due to adaptation to met. state of illness:
  1. Free T4 us. nl/high but low in severely ill pts
  2. TSH us. nl in mild-mod illness often transiently high (days-wks) during recovery from severe illness.
  1. Drugs which alter TFTs
  1. Dopamine--lowers TSH
  2. Corticosteroids--impairs T4-->T3 conversion in periperal tissues; lowers TSH
  3. Phenytoin--lowers Free T4; TSH us. nl
  4. Carbamazepine--lowers Free T4
  5. Rifampin--lowers Free T4
  6. Propanolol--impairs T4-->T3 conv. in periphery
  7. amiodarone--impairs T4-->T3 conv. in periphery
  8. Li: mild-mod tox causes TSH with nl free T4 (2? T4 catabolis); severe causes TSH and T4