Interpretation of Laboratory Thyroid Function Tests
Interpretation of Laboratory Thyroid Function Tests
Thyroid function tests are frequently ordered in both the inpatient and outpatient settings. A basic knowledge of thyroid-hormone physiology is helpful in interpretation of laboratory results.
Thyroxine (T4) is released from the thyroid gland along with small amounts of triiodothyronine (T3) and thyroglobulin under the guidance of thyroid-stimulating hormone (TSH). Secretion of TSH is principally regulated by circulating levels of thyroid hormones (via a negative-feedback loop) and hypothalamic thyrotropin-releasing hormone (TRH). More than 99% of the thyroid hormones are bound to proteins, including thyroid-binding globulin (TBG), thyroxine-binding prealbumin, and albumin. Less than 1% are free, unbound hormones that make up the biologically active fraction, and they are not generally influenced by thyroid-binding protein abnormalities. Thyroxine is tightly bound to TBG, whereas T3 is less tightly bound to TBG but more tightly bound to thyroxine-binding prealbumin and albumin. The majority of serum T3 levels (over 75%) come from peripheral conversion of T4. In nonthyroidal illness, T4 conversion to T3 is reduced and conversion to reverse T3 (rT3) is enhanced.
Introduction
Thyroid function tests are frequently ordered in both the inpatient and outpatient settings. A basic knowledge of thyroid-hormone physiology is helpful in interpretation of laboratory results.
Thyroxine (T4) is released from the thyroid gland along with small amounts of triiodothyronine (T3) and thyroglobulin under the guidance of thyroid-stimulating hormone (TSH). Secretion of TSH is principally regulated by circulating levels of thyroid hormones (via a negative-feedback loop) and hypothalamic thyrotropin-releasing hormone (TRH). More than 99% of the thyroid hormones are bound to proteins, including thyroid-binding globulin (TBG), thyroxine-binding prealbumin, and albumin. Less than 1% are free, unbound hormones that make up the biologically active fraction, and they are not generally influenced by thyroid-binding protein abnormalities. Thyroxine is tightly bound to TBG, whereas T3 is less tightly bound to TBG but more tightly bound to thyroxine-binding prealbumin and albumin. The majority of serum T3 levels (over 75%) come from peripheral conversion of T4. In nonthyroidal illness, T4 conversion to T3 is reduced and conversion to reverse T3 (rT3) is enhanced.
