Thyroid-Binding Globulin
Updated: May 09, 2022
Author: Benjamin Daniel Liess, MD; Chief Editor: Sridevi Devaraj, PhD, DABCC, FAACC, FRSC, CCRP
Thyroid-binding globulin (TBG) is produced in the liver and is a circulating protein that reversibly binds to thyroid hormones 3,5,3’-triiodothyronine (T3) and thyroxine (T4) and carries them in the bloodstream.[1]
The reference ranges for TBG are shown in Table 1.[2]
Table 1. (Open Table in a new window)
| Age | Male (mg/dL) | Female (mg/dL) |
|---|---|---|
| 1-5 days | 2.2-5.9 | 2.2-5.9 |
| 1-11 months | 3.1-5.6 | 3-5.6 |
| 1-9 years | 2.5-5 | 2.5-5 |
| 10 to 19 years | 2.1-4.6 | 2.1-4.6 |
| Over age 20 years | 1.2-2.5 | 1.4-3 |
With oral contraceptive use, the reference range is 1.5-5.5 mg/dL, while in the third trimester of pregnancy, it is 4.7-5.9 mg/dL.[2]
An increase in TBG may result in an increase in total T4 and T3 without an increase in hormone activity in the body. If additional thyroid hormone testing is indicative of hypo- or hyperthyroidism without any symptoms, TBG levels become more relevant. TBG levels can artificially suggest states of hypothyroidism or hyperthyroidism. Various nonthyroidal illnesses, medications, high estrogen states, and even prematurity can mimic hypothyroidism as a result of misleading laboratory findings. Increased TBG levels may be due to hypothyroidism, liver disease, and pregnancy. Decreased TBG levels may be due to hyperthyroidism, renal disease, liver disease, severe systemic illness, Cushing syndrome, medications, and malnutrition. Thus, the entire laboratory data collection evaluating thyroid function should be considered along with the current state of health.[3, 4]
See the list below:
Specimen: Blood
Container: Serum separator tube
Collection method: Routine venipuncture[5]
TBG is not typically a part of a panel and should be ordered specifically.
Thyroid-binding globulin (TBG) is produced in the liver and is a circulating protein that reversibly binds to thyroid hormones 3,5,3’-triiodothyronine (T3) and thyroxine (T4) and carries them in the bloodstream. T3 and T4 do not circulate the blood stream freely at high levels only 0.3% or less. TBG, transthyretin (TTR or prealbumin), and albumin are the 3 proteins that carry thyroid hormone in the body. TBG transports nearly 75% of serum T4 and T3 and has a 10-fold greater affinity for T4 than for T3. Circulating TBG is typically only 25% saturated with T4.[4]
TBG levels are important to consider if a patient seems to have normal thyroid function but abnormal levels of T4 or T3—in other words, when total thyroid hormone levels do not correlate with a patient’s thyrometabolic status. An increase in TBG may result in an increase in total T4 and T3 without an increase in hormone activity in the body. Increased TBG levels may be due to hypothyroidism, liver disease, and pregnancy. In cases of high TBG, more thyroid hormone will be bound, decreasing free hormone in the blood. This will lead to the increase of TSH, and more thyroid hormones will be produced. The total thyroid hormone level will be high, free thyroid hormone levels will be normal and TBG will be high.
Low levels of TBG may be found in cases of genetic TBG deficiency. Those patients may be misdiagnosed as hyperthyroid or hypothyroid but have normal thyroid levels and warrant no treatment. Decreased TBG levels may also be due to hyperthyroidism, renal disease, liver disease, severe systemic illness, Cushing syndrome, medications, and malnutrition. If TBG levels are low, total thyroid levels will be low and free thyroid hormone levels may be normal or low. If a patient is truly hypothyroid or hyperthyroid, then TBG levels will be normal and the total thyroid hormone levels will be low or high, respectively.
TSH, free T4, free T3, total T4, and total T3 levels may be ordered in addition to evaluate a patient‘s thyroid function.