Thyroid – Part 7 – Triiodothyronine Total, T3 Total

Sample

• This test is done on the serum of the patient.

Precaution

1. Ask the history of radioisotopes if given before the test, because it will alter the result.
2. In pregnancy T3 values are increased.
3. Some of the drugs increased the T3 level like estrogens, oral contraceptives, and methadone.
4. Some of the drugs decreased the T3 level like anabolic steroids, phenytoin (Dilantin), androgens, Inderal, salicylates, and reserpine.

Indications

1. T3 evaluate the thyroid function.
2. T3 is used to diagnose hyperthyroidism
3. T3 is used to monitor the therapy.

Pathophysiology

1. Tyrosine combine with organic iodine form monoiodotyrosine.
2. Monoiodotyrosine with another molecule of iodine  Diiodotyrosine.
3. Monoiodotyrosine + diiodotyrosine = Triiodothyronine ( T3 ).
   

   Thyroid hormone metabolism

 Metabolism of thyroid hormones

Formation of T 3

4. T3 has three atoms of iodine as compared to T4 which has 4 atoms of iodine.
   1. A large amount of T3 is formed in the liver by T4.
   2. T3 is less stable than T4 because it is less tightly bound to serum protein.
5. 7 to 10% of the thyroid hormone is T3.
6. 70 % of T3 bound to thyroglobulin ( TBG ) and albumin.
   1. The very small amount is free T3 which is biologically active.

7. Free T3 is metabolically active.
8. Because of the conversion of T4 to  T3 in the liver, so the T3 level is less useful for hypothyroidism.
   1. A T3 level helps to diagnose Hyperthyroidism.
9. If there are raised T3 along with T4  indicate Hyperthyroidism.
10. 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. 1 -5 years = 105  to 269 ng/dL
  2. 6 to 10 years = 94 to 241 ng/dL
  3. 16 to 20 years = 80 to 210 ng/dL
• Adult
  1. 20 to 50 years = 70 to 204 ng/dL
  2. 50 to 90 years = 40 to 181 ng/dL
• Pregnancy
  1. Last 5 months = 116 to 247 ng/dL
     1. To convert into SI unit x 0.0154 = nmol/L

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:

1. primary hyperthyroidism like :
   1. grave’s disease.
   2. Toxic thyroid adenoma.
2. Acute thyroiditis. In the early stages, the thyroid produces more T3.
3. Ectopic thyroid tissue e.g. Struma ovarii.
4. Increased Thyroid binding globulin, seen in pregnancy, Hepatitis, and congenital hyperproteinemia.

Decreased level of T3:

1. Hypothyroidism is seen in :
   1. Cretinism.
   2. Surgical ablation.
   3. Myxedema.
2. Hypothalamic failure.
3. Nephrotic syndrome.
4. Iodine insufficiency.
5. Pituitary insufficiency.
6. Renal failure.
7. Cirrhosis.
8. advanced cancer.
9. hepatic diseases.

• Please see more details in thyroid function and T3 Free

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