Thyroid – Part 8 – T3 Free , Free Triodothyronine

Sample

1. This is done on the serum of the patient.
2. A non-fasting blood sample can be taken.
3. Plasma (heparin or EDTA) can be used.
4. The sample is stable 7 to 14 days at room temperature or at 2 to 8 °C.

Precautions

1. Avoid hemolyzed or lipemic samples.
2. Centrifuge turbid sample before performing the test.

Indication

1. Used to evaluate the thyroid function.
2. It is done to rule out T3 toxicosis.
3. This is done to clarify the protein binding abnormalities.
4. This is also done to monitor the therapy.

Pathophysiology

1. This is the free T3 which is unbound to the protein.
2. 99 % of T3 is bound to carrier proteins.
3. The main transport protein is thyroxine-binding globulin (TBG).
4. Also, this is bound to albumin and prealbumin.

5. Only the free (unbound) portion of triiodothyronine (free T3) is believed to be responsible for the biological action.
6. As the concentrations of the carrier proteins are altered in many clinical conditions, such as pregnancy, etc..
7. In normal thyroid function, as the concentrations of the carrier proteins changes, the total triiodothyronine level also changes.

8. But the free T3 concentration remains constant even in the abnormal thyroid gland.
9. Measurements of free T3 concentrations, therefore, correlate more reliably with your clinical status than total T3 levels.
10. The total T3 level may be increased in :
    1. Pregnancy.
    2. Oral contraceptives.
    3. estrogen therapy.
11. In the above conditions free T3 level remains normal.
12. T3 half-life is only one day as compare to thyroxine (T4) which is one week.

normal

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

• 1 to 3 days = 100 to 740 ng/dL
• 1 to 11 months = 105 to 245 ng/dL
• 1 to 5 years = 105 to  270 ng/dL
• 6 to 10 years = 95 to 240 ng/dL
• 11 to 15 years = 80 to 215 ng/dL
• 16 to 20 years = 80 to 210 ng/dL
• 20 to 50 years = 75 to 220 ng/dL
• >50 years = 40 to 180 ng/dL

• Adult = 260 to 480 pg/dL

                                            Diagnostic value of Free T 3 in relation to TSH

Increased Free T3 values are seen in:

1. T3 toxicosis.
2. Hyperthyroidism.
3. Peripheral resistance syndrome.

Decreased Free T3 values are seen in:

1. Hypothyroidism.
2. Pregnancy in the third trimester.

Please see more details in Total T3 and Thyroid function

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