Thyroid – Part 7 – Triiodothyronine Total, T3 Total
Sample
- This test is done on the serum of the patient.
Precaution
- Ask 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.
Indications
- T3 evaluate the thyroid function.
- T3 is used to diagnose hyperthyroidism
- T3 is used to monitor the therapy.
Pathophysiology
- Tyrosine combine with organic iodine form monoiodotyrosine.
- Monoiodotyrosine with another molecule of iodine Diiodotyrosine.
- Monoiodotyrosine + diiodotyrosine = Triiodothyronine ( T3 ).
Metabolism of thyroid hormones
Formation of T 3
- 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 bound to thyroglobulin ( TBG ) and albumin.
- The very small amount is free T3 which is biologically active.
- Free T3 is metabolically active.
- Because of the conversion of T4 to T3 in the liver, so the T3 level is less useful for hypothyroidism.
- A T3 level helps to diagnose Hyperthyroidism.
- If there are raised T3 along with T4 indicate Hyperthyroidism.
- A rare form of hyperthyroidism called T3 toxicosis where T3 is raised and T4 normal.
Normal
Source 1
T3 Total
- Cord blood (>37 weks) = 5 to 141 ng/dL
- 1 to 3 days = 100 to 740 ng/dL
- 1 to 11 months = 105 to 245
- Children
- 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
- Adult
- 20 to 50 years = 70 to 204 ng/dL
- 50 to 90 years = 40 to 181 ng/dL
- Pregnancy
- 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
Source 1
Free T3
- Cord blood (>37 weeks ) = 15 to 391 pg/dL
- Child and adult = 260 to 480 pg/dL (4.0 to 7.4 pmol/L)
- Pregnancy
- First trimester = 211 to 383 pg/dL
- Second and third trimester = 196 to 338 pg/dL
Source 2
T3 total
Age | ng/dL |
1 to 3 days | 100 to 740 |
1 to 11 months | 105 to 245 |
Child 6 to 10 years | 95 to 240 |
11 to 15 years | 80 to 215 |
Adult 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 varies from lab to lab)
Increased level of 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, seen in pregnancy, Hepatitis, and congenital hyperproteinemia.
Decreased level of T3:
- Hypothyroidism is seen in :
- Cretinism.
- Surgical ablation.
- Myxedema.
- Hypothalamic failure.
- Nephrotic syndrome.
- Iodine insufficiency.
- Pituitary insufficiency.
- Renal failure.
- Cirrhosis.
- advanced cancer.
- hepatic diseases.
- Please see more details in thyroid function and T3 Free