Thyroid

Excesses or deficiencies of thyroid hormones, including free T3, total T3, free T4, total T4 and TSH, cause many well-recognized conditions and diseases. Traditional testing of TSH level may miss some cases of hypothyroidism (insufficient thyroid hormone production) or hyperthyroidism excess thyroid hormone production). For example, approximately three percent of ‘healthy’ women over age 50 will test positive for hypothyroidism as indicated by measurement of TSH. Another significant percentage of ‘healthy’ women also show evidence of low thyroid function, when tested for free T3 and free T4 levels.

T3 Total

T3 Total – is produced primarily by the 5-monodeionation of thyroxine (T4) in extrathyroidal tissues. T3 production within the thyroid gland occurs at a rate that is three times lower than the rate for T4. In the serum, T3 is bound to thyroxine binding globulin (TBG), albumin and prealbumin. Although the circulating levels of T3 and free T3 are several times lower than for total and free T4, thyroid hormone receptor affinity is much greater for T3. Therefore, the biological potency of T3 is greater than that of T4. Measurement of total serum T3 concentrations is a standard and well-validated test of thyroid gland function.

T4 Total

T4 Total – is the primary active hormone synthesized within the follicular cells of the thyroid gland. The production of T4 is approximately three times that of triiodothyronine (T3). In the plasma, thyroxine binding globulin (TBG) binds 70 percent of T4 while 15-25 percent of T4 is bound to transthyretin, 5-15 percent to albumin, and a small fraction is bound to erythrocytes. Less than 0.1 percent of the total T4 circulates in the free unbound form. Measurement of total serum T4 concentrations is a standard and well-validated test of thyroid gland function.

TSH

TSH – provides an overall assessment of the patient’s thyroid function and is an excellent indicator of thyroid health. TSH results along with the other thyroid tests can assist in the diagnosis of hypothyroidism and hyperthyroidism. TSH acts in a negative feedback loop with T3 and T4. When T3 and T4 levels are increased, production of TSH goes down. A break in this negative feedback loop results in poor correlation between the levels of T3, T4 and TSH, and is an indication of possible pathology.

T3 Free

T3 Free – in the serum, the majority of triiodothyronine (T3) is bound to the major binding proteins: thyroxine binding globulin (TBG), albumin and prealbumin. A small fraction (less than 0.3 percent) circulates as unbound or free T3 and represents the physiologically active form. The concentration of free T3 in the blood is regulated by a negative feedback mechanism involving thyroid stimulating hormone (TSH). Changes in the circulating levels of TBG will result in a proportional increase or decrease in the concentration of total T3. Measurement of serum free T3 levels is unaffected by the changes in T3 binding protein levels and therefore correlates well with the functional thyroid state in most individuals.

T4 Free

T4 Free – in the serum, the majority of thyroxine (T4) in the blood is bound to the major binding proteins: thyroxine binding globulin (TBG), albumin and prealbumin. A small fraction (less than 0.1 percent) circulates as unbound or free T4 and represents the physiologically active T4. T4 is also the major precursor to triiodothyronine (T3). The concentration of free thyroid hormones in the blood is regulated by a negative feedback mechanism involving thyroid stimulating hormone (TSH). Changes in the circulating levels of TBG will result in a proportional increase or decrease in the concentration of total T4. Measurement of serum free T4 is unaffected by the changes in T4 binding protein levels and therefore correlates well with the functional thyroid state in most individuals.