Triiodothyronine

Triiodothyronine (T3), molecular formula C15H12I3NO4, by far the most biologically active thyroid hormone, represents the bound triiodothyronine. fT3 represents the free triiodothyronine. rT3 represents the reverse triiodothyronine.

Triiodothyronine is the actual, biologically active thyroid hormone (exceeds T4, also known as thyroxine, in its biological activity by a multiple of up to 100 (?) times), which is produced by monodeiodination (outer ring deiodination) from its prohormone thyroxine (T4). In serum, T3 is 99% coupled to binding proteins (transport proteins). Only 1% is present in free, unbound form (fT3). The half-life in serum is 12-18h.

The two iodine-containing thyroid hormones T3 and T4 differ only in the number of bound iodine atoms. These are 3 iodine atoms for triiodothyronine and 4 iodine atoms for thyroxine (hence the acronyms T3 and T4).

Free triiodothyronine (fT3) and T3 (triiodothyronine) play an important role in the diagnosis of thyroid function and in the assessment of thyroid diseases.

Thyroid gland diagnostics include other laboratory values such as T4 (tetraiodothyronine, thyroxine) or fT4 (free T4) and TSH (thyroid stimulating hormone).

The free triiodothyronine (fT3) can be measured in the blood, as can (the T3 bound to thyroglobulin).

Test material: serum, plasma

For adults the following reference values for fT3 and T3 apply:

reference values:

• Healthy adults: 1.4-2.80 nmol/l
• Children have higher values, older adults lower values (skewed normal distribution)
• The serum concentration shows a diurnal rhythm with a decrease in concentration during the day and an increase at night.

Elevated (fT3 or) T3 values indicate hyperthyroidism; elevated (fT3 and) T3 values are also possible by taking thyroid medication containing T3.

Reduced (fT3 or) T3 values are an indication of hypothyroidism.

Incorrectly increased or decreased T3 values can result from disturbances in TGB production (see thyroxine below).

Enzyme immunological and radioimmunological methods are available for the detection of triiodothyronine in serum and for therapy control.

The T3 isomer, in which only 1 iodine atom is bonded to the inner benzene ring and 2 iodine atoms to the outer ring, is called "reverse triiodothyronine", (rT3). Reverse triiodothyronine is a breakdown product of thyroxine and is a biologically inactive form of triiodothyronine (see there).

1. Böhm BO (2018) Thyroid hormones. In: Neumeister B et al. (Eds) Clinical guide to laboratory diagnostics. Elsevier GmbH S. 297-298