Understanding Thyroid Function Tests: TSH, T4, T3 Uptake & More

Understanding Thyroid Function Tests: TSH, T4, T3 Uptake & More

Question. Discuss thyroid function tests.

Answer. A laboratory thyroid profile includes measurement of

• Serum T4
• Resin T3 uptake
• T4 binding ratio
• Free T4 index
• Thyroid-stimulating hormone.

“Understanding thyroid function tests through FAQs: TSH, T4, T3 uptake explained”

Serum t4

• Measured total serum T4 reflects hormone that both binds to thyroid­binding globulin and free hormone.
• The normal range of serum T4 is 5–11.5 µg/DL. Hence, changes in either free hormone level or concentration in thyroid­binding globulin will alter total serum T4 but not affect free hormone level.
• Therefore, increased secretion of thyroid­binding globulin increases total serum T4, whereas a decrease in TBG lowers total serum T4.

Understanding Thyroid Function Tests: TSH, T4, T3 Uptake & More

Resin T3 Uptake

• It reflects the thyroid-binding globulin concentration and is reported as a percentage, i.e. 25 to 35%.
• A present amount of radioactive T3 is added to the patient’s sample of serum.
• Radioactive T3 binds to all liable binding sites on thyroid-binding globulin and leftover is bound to resin and is measured.

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T4 Binding ratio

• The resin T3 uptake is converted to the T4 binding ratio by dividing the measured resin T3 uptake by reference serum mean resin T3 uptake which is 30%.
• The resin T3 uptake and T4 binding ratio are low and high in hypothyroidism and hyperthyroidism, respectively.

Free t4 Index

• It represents a calculated free T4 hormone concentration. It is calculated by multiplying the total serum T4 by the T4 binding ratio.
• The free T4 index is low and high in hypothyroidism and hyperthyroidism.

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Thyroid-stimulating hormone (TSH)

• It is secreted from the anterior pituitary and has a negative feedback relationship with circulating T4 and T3.
• It is the single best test for diagnosing primary hypothyroidism and hyperthyroidism.
• In the former condition, serum TSH is increased and in the latter condition, it is decreased.