7.2 Thyroid Evaluation
Key Features
Thyroid function tests are common procedures performed to determine how well the thyroid is functioning.
Evaluation may include blood tests and imaging (ultrasound), nuclear thyroid scans, fine-needle aspiration and functional stimulation tests. Recently, molecular testing of thyroid cytology has been used in cases of indeterminate thyroid nodules.
Thyroid Function Tests
T4 and T3 (Total)
Both T4 (> 99%) and T3 (~ 98%) are tightly bound to transport proteins in the plasma: thyroxine-binding globulin (TBG), thyroxine-binding prealbumin (TBPA), or albumin. Therefore, only 1% of the thyroid hormones are in the metabolically active “free” state. Assays of “total” T4 or T3 measure mainly the protein-bound hormone (plus the unbound or “free” hormone). Values may vary with conditions that affect protein concentration ( Table 7.1 ). Because drugs and illness can alter concentrations of binding proteins, it is necessary to estimate free hormone concentrations.
Thyroid-Stimulating Hormone
The thyroid-stimulating hormone (TSH) assay is the screening test of choice for thyroid function. The very sensitive TSH assay (it measures TSH into very low ranges between 0.01 and 0.001 mU/L) is currently in wide use.
Free T4 and T3 (FT4 and FT 3)
These assays measure the unbound, metabolically active thyroid hormones in the circulation (~ 0.0003 to total). Free thyroid hormone tests fall into two main categories: equilibrium dialysis (not affected by TBG abnormalities) and analogue assays (affected by protein binding but still superior to total T3 and T4 assays).
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