What does it mean when I have low T3 uptake but normal TSH and T4?
You may have a low T3 uptake but normal TSH and T4 for a few reasons.
For example, you can have low T3 Syndrome. It is also called euthyroid sick syndrome and occurs in people with a disease that does not affect the thyroid [1].
Another problem that low T3 uptake indicates is a problem with thyroid hormone-binding proteins. Again, the possible problem is not with the thyroid [2].
The problem with the thyroid-binding proteins can originate from genetic disorders, like thyroxine-binding globulin deficiency or familial dysalbuminemic hypertriiodothyroninemia [3, 4].
Also, some medications and diseases can lead to a thyroid-binding protein abnormality. Examples of such drugs or conditions are
- amiodarone [5],
- furosemide and fenclofenac [6],
- chronic renal failure [7],
- severe caloric restriction or fasting [8],
- malnutrition [9].
Moreover, having elevated bilirubin and non-esterified fatty acids can lead to abnormal thyroid hormone binding [6].
It is also possible that you have a subclinical thyroid dysfunction. However, this condition can be only responsible for small abnormalities in the T3 uptake [10].
Another possibility is that the low T3 uptake results from measurement variability. It depends on the material used in the lab to measure the uptake. Some labs use albumin, and others, for example, charcoal [11].
What are the T3 and T4?
T3 and T4 are thyroid hormones produced by the thyroid gland in the neck. They are crucial for regulating human metabolism and growth [12].
Despite production at the same gland, the secretion ratio is very different. More than 90% of all secreted thyroid hormones are T4. This is a strategic adaptation of the thyroid [13].
T4, or chemically thyroxin, acts more as a prohormone. This means that T4 becomes an active hormone after conversion [13, 14].
The conversion of T4 to T3 happens in the body's tissues after T4 binds to the thyroid hormone receptor [15].
The product of this conversion is the T3 hormone or triiodothyronine, which is more biologically potent than T4 [13, 14].
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What is TSH?
TSH stands for thyroid stimulating hormone. It is also known as thyrotropin and is produced in the pituitary gland in the head [18, 19].
The role of this hormone is the simulation of the T3 and T4 production and secretion [20].
The release and production of TSH are regulated by a loop mechanism. Production decreases in inverse relationship to the increase in the T3 and T4 concentrations in the blood. When the levels of the thyroid hormone fall, the opposite occurs, and TSH production and release increase [21, 22].
TSH is part of the laboratory assessment of thyroid function [20]. It helps to determine whether the thyroid is normal, overreactive, or underactive [21].
Should I visit a doctor with Low T3 uptake but normal TSH and T4?
It may not be urgent, but you should consult such a reading with a doctor [23]. The best specialist to reach out to with thyroid-related questions is an endocrinologist.
If the test is suspicious, the endocrinologist will repeat it. If the test looks good, the doctor will confirm that nothing is wrong. It can happen when there are no other noticeable symptoms related to the thyroid.
However, it is important to remember that thyroid function is very complex. Sometimes, one test is not enough to confirm or reject a diagnosis [23].