Thyroid:- Part 7 – Triiodothyronine Total, T3 Total
March 28, 2023Chemical pathologyLab Tests
Triiodothyronine Total (T3)
Sample for Triiodothyronine Total (I3)
- This test is done on the serum of the patient.
Precaution for Triiodothyronine Total
- Ask about the history of radioisotopes if given before the test because it will alter the result.
- In pregnancy, T3 values are increased.
- Some of the drugs increased the T3 level, like estrogens, oral contraceptives, and methadone.
- Some of the drugs decreased the T3 level, like anabolic steroids, phenytoin (Dilantin), androgens, Inderal, salicylates, and reserpine.
- It is not advised for the diagnosis of hypothyroidism because the decreased value has minimal significance.
- It decreases in healthy older persons when Free T4 is normal.
Indications for Triiodothyronine Total (T3)
- T3 evaluates the thyroid function.
- T3 is used to diagnose hyperthyroidism.
- T3 is used to monitor the therapy in case of hyperthyroidism.
- Diagnosis of T3 thyrotoxicosis (TSH is suppressed, and T4 is normal).
- It is advised when Free T4 is borderline raised.
- In the cases of unexplained atrial fibrillation.
- It is advised to monitor the T4 replacement therapy. It is better than T4 and Free T4, but TSH is still a better test.
- It is advised in Grave’s disease to evaluate antithyroid drug effects.
- It is a better indicator of thyrotoxicity in hyperthyroidism.
Definition of Triiodothyronine Total (T3)
- Thyroxine (T4) is converted to Triiodothyronine (T3) in the peripheral tissues.
- ∼20% is synthesized by the follicular cells.
- Most T3 is bound to proteins, and a very small amount of 0.3% is free (unbound) and biologically active.
Triiodothyronine (T3) formation:
- Tyrosine combines with organic iodine to form monoiodotyrosine.
- Monoiodotyrosine with another molecule of iodine Diiodotyrosine.
- Monoiodotyrosine + diiodotyrosine = Triiodothyronine ( T3 ).
- T3 has three atoms of iodine as compared to T4, which has 4 atoms of iodine.
- A large amount of T3 is formed in the liver by T4.
- T3 is less stable than T4 because it is less tightly bound to serum protein.
- 7% to 10% of the thyroid hormone is T3.
- 70% of T3 is bound to thyroglobulin (TBG) and albumin.
- The very small amount is free T3 which is biologically active.
Metabolism of thyroid hormones (T4 and T3):
- Because of the conversion of T4 to T3 in the liver, the T3 level is less useful for hypothyroidism.
- A T3 level helps to diagnose Hyperthyroidism.
- If there are raised T3 along with T4 indicates Hyperthyroidism.
- A rare form of hyperthyroidism is called T3 toxicosis, where T3 is raised and T4 normal.
- Free T3 is metabolically active.
Reverse T3 (rT3)
- It is advised to distinguish low T3 sick thyroid.
- It is a hormonally inactive form of T3.
- It is usually increased in hyperthyroidism.
- It is usually decreased in hypothyroidism.
Normal Triiodothyronine Total (T3)
Triiodothyronine (T3) Total
- Cord blood (>37 weeks) = 5 to 141 ng/dL
- 1 to 3 days = 100 to 740 ng/dL
- 1 to 11 months = 105 to 245
- 1 -5 years = 105 to 269 ng/dL
- 6 to 10 years = 94 to 241 ng/dL
- 16 to 20 years = 80 to 210 ng/dL
- 20 to 50 years = 70 to 204 ng/dL
- 50 to 90 years = 40 to 181 ng/dL
- Last 5 months = 116 to 247 ng/dL
- To convert into SI unit x 0.0154 = nmol/L
- Last 5 months = 116 to 247 ng/dL
Another source 1
- Cord blood (>37 weeks ) = 15 to 391 pg/dL
- Child and adult = 260 to 480 pg/dL (4.0 to 7.4 pmol/L)
- First trimester = 211 to 383 pg/dL
- Second and third trimesters = 196 to 338 pg/dL
Another source 2
|1 to 3 days||100 to 740|
|1 to 11 months||105 to 245|
|Children 6 to 10 years||95 to 240|
|11 to 15 years||80 to 215|
|Adults 16 to 20 years||80 to 210|
|20 to 50 years||75 to 220|
|Adult > 50 years||40 to 180|
- According to another source, the values are:
- Adult = 80 to 200 ng/dL.
- Adolescent (12 to 23 years) = 82 to 213 ng/dL.
- Children (1 to 14 years) = 105 to 245 ng/dL.
- Pregnancy = 116 to 247 ng/dL.
- (Values vary from lab to lab)
Increased level of Triiodothyronine Total (T3):
- Primary hyperthyroidism like :
- Grave’s disease.
- Toxic thyroid adenoma.
- Acute thyroiditis. In the early stages, the thyroid produces more T3.
- Ectopic thyroid tissue, e.g., Struma ovarii.
- Increased Thyroid binding globulin is seen in pregnancy, Hepatitis, and congenital hyperproteinemia.
Decreased level of Triiodothyronine Total (T3):
- Hypothyroidism is seen in :
- Surgical ablation.
- Hypothalamic failure.
- Nephrotic syndrome.
- Iodine insufficiency.
- Pituitary insufficiency.
- Renal failure.
- Advanced cancer.
- Hepatic diseases.
Triiodothyronine T3 interpretations:
|TSH- secreting tumors||Increased|
Questions and answers:
Question 1: What is reverse T3?
Reverse T3 (rT3) is inactive form of T3.
Question 2: Is there any value of T3 in hypothyroidism?
No, it has less value in hypothyroidism.
- Please see more details on thyroid function and T3 Free.