Thyroid Testing and Immunoassays

Automated Immunoassays

• Highly specific with no cross-reactivity between T3 and T4 hormones.
• T4 measurement serves as a screening test for thyroid diseases.
• T3 measurement helps diagnose uncommon instances of hyperthyroidism, particularly when T4 levels are low or normal, while T3 levels are elevated.
• Monitoring of thyroid replacement therapy is essential for effective management.

Thyroxine-Binding Globulin (TBG)

• Abnormal total T3 or T4 levels warrant TBG level checks.
• T4 RIA (Radioimmunoassay) and T3RU (T3 Resin Uptake) tests measure TBG and hormone attachment.
• T3RU provides an indirect estimate of the plasma thyroid-binding proteins' binding capacity.
• Increased T3RU indicates more unoccupied binding sites on TBG and higher thyroid hormone levels.

Determination of Results

• High radiolabeled T3 radioactivity signals elevated hormone levels in the blood and fewer TBG binding sites.
• Low radioactivity suggests low hormone levels with many TBG binding sites.
• Factors affecting hormone uptake include TBG levels in serum.
• Estrogen raises TBG levels, whereas androgens and glucocorticoids lower them.
• T3 uptake is diminished during pregnancy.

Free Hormone Level

• Free hormone levels are challenging to measure due to their small quantities yet are physiologically active.
• Free T4 measurement can be achieved through equilibrium dialysis (requires special expertise) or RIA (susceptible to interferences).
• Free Thyroxine Index (FTI) is the most commonly utilized method, calculated as measured T4 multiplied by T3 uptake.
• FTI is unitless and adjusts for both hormone level and TBG binding ability, with a normal range of 1.2-4.2.
• FTI is elevated in hyperthyroidism but reduced in hypothyroidism.

Stimulation of TSH

• TSH measurement is performed using immunoassays with monoclonal antibodies, facing potential cross-reactivity with FSH, LH, and hCG.
• Low TSH levels indicate hyperthyroidism or secondary hypothyroidism, while high levels confirm primary hypothyroidism.

Clinical Significance: Hypothyroidism

• Characterized by insufficient thyroid hormone availability to tissues.
• Treated predominantly with levothyroxine for hormone replacement.
• Symptoms include bradycardia, weight gain, rough skin, cold intolerance, and mental sluggishness.

Primary Hypothyroidism

• Mostly arises from iodine deficiency, resulting in low T3 and T4 with elevated TSH levels.
• Other causes include surgical removal of the thyroid, radioactive iodine treatment for hyperthyroidism, radiation exposure, and certain medications like lithium.

Secondary and Tertiary Hypothyroidism

• Secondary hypothyroidism results from pituitary insufficiency, showing low levels of T3, T4, and TSH.
• Tertiary hypothyroidism stems from hypothalamic insufficiency, also presenting with low T3, T4, and TSH.

Clinical Significance: Hashimoto’s Disease

• Characterized by inflammation, leading to damage of the thyroid gland.