Triiodothyronine Total (T3)

What sample is needed for Triiodothyronine Total (T3)?

• This test is done on the patient’s serum.

What are the precautions for triiodothyronine total (T3)?

1. Ask about 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.
5. It is not advised for the diagnosis of hypothyroidism because the decreased value has minimal significance.
6. It decreases in healthy older persons when Free T4 is normal.

What are the indications for Triiodothyronine total (T3)?

1. T3 evaluates the thyroid function.
2. T3 is used to diagnose hyperthyroidism.
3. T3 is used to monitor the therapy in case of hyperthyroidism.
4. Diagnosis of T3 thyrotoxicosis (TSH is suppressed, and T4 is normal).
5. It is advised when Free T4 is borderline raised.
6. In the cases of unexplained atrial fibrillation.
7. It is advised that T4 replacement therapy be monitored. It is better than T4 and Free T4, but TSH is still a better test.
8. It is advised in Grave’s disease to evaluate antithyroid drug effects.
9. It is a better indicator of thyrotoxicity in hyperthyroidism.

How will you define triodothyronine total (T3)?

1. Thyroxine (T4) is converted to Triiodothyronine (T3) in the peripheral tissues.
   1. ∼20% is synthesized by the follicular cells.
   2. Most T3 is bound to proteins; a small amount, 0.3%, is free (unbound) and biologically active.

What is the mechanism of formation of Triiodothyronine (T3)?

1. Tyrosine combines with organic iodine to form monoiodotyrosine.
2. Monoiodotyrosine, along with another molecule of iodine, will form  Diiodotyrosine.
3. Monoiodotyrosine + diiodotyrosine = Triiodothyronine ( T3 ).

 What is the mechanism of  Metabolism of thyroid hormones (T4 and T3)?

1. The liver converts T4 to T3, so the T3 level is less useful for hypothyroidism.
   1. A T3 level helps to diagnose Hyperthyroidism.
2. If T3 and T4 are raised, it indicates hyperthyroidism.
3. A rare form of hyperthyroidism is called T3 toxicosis, where T3 is raised and T4 normal.
4. Free T3 is metabolically active.

How will you discuss Reverse T3 (rT3)?

1. It is advised to distinguish low T3 sick thyroid from true hypothyroidism.
2. It is a hormonally inactive isomer of T3.
3. It is usually increased in hyperthyroidism.
4. It is usually decreased in hypothyroidism.

What is normal Triiodothyronine Total (T3)?

Source 1

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
• 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

Another 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 trimesters = 196 to 338 pg/dL

Another source 2

T3 total

• 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)

What are the causes of increased levels of  Triiodothyronine Total (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 is seen in pregnancy, Hepatitis, and congenital hyperproteinemia.

What are the causes of decreased levels of  Triiodothyronine Total (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.

How will you interpret Triiodothyronine T3?

How will you differentiate different thyroid diseases?

Questions and answers:

• Please see more details on thyroid function and T3 Free.