T4 total: Measurement of serum total T4 is the most useful routine thyroid function test in the diagnosis of thyrotoxicosis or hypothyroidism. The calculation of a free thyroxine index (FTI) adjusts the serum thyroxine for changes in the serum proteins where total serum T4 may not give a good index of thyroid function, e.g. during oral contraceptive therapy or pregnancy a serum thyroxine is often significantly elevated, but the calculated FTI is normal. Conversely, in patients with subnormal serum proteins the serum thyroxine is often low but the FTI is normal.
T3 total: The measurement of the total serum T3 has a very limited use in patients where a diagnosis of thyrotoxicosis is suspected but the serum T4 is high normal or only marginally elevated. Total serum triiodothyronine levels fall with age so that the value may be in the high normal range in thyrotoxicosis in the elderly. Calculation of a "free T3 index" using a measurement of thyroxine binding globulin may also be necessary.
•The diagnosis of hypothyroidism.
•To separate primary thyroid failure from hypopituitarism. In thyroid deficiency the TSH is elevated whereas in pituitary-hypothalamic disease the TSH is usually normal or subnormal.
•In association with Thyrotrophin Releasing Hormone stimulation tests.
•Diagnosis of thyrotoxicosis.
•Assessment of replacement thyroxine therapy.
Ambient (8 - 24 degrees Celsius)
Minimum 1.0 mL serum, ideally 1.5 mL serum. Fridge 4°C
Transport to lab ambient in leak proof tubes
T3: Elevated values – thyrotoxicosis, thyroxine or amiodarone therapy, and severe illness.
Subnormal values – hypothyroidism, hypopituitarism, lithium therapy, and severe illness.
Note: The total serum triiodothyronine level in particular may be subnormal in patients with chronic or acute severe non-thyroidal illness and in malnutrition.
1. In primary hypothyroidism the TSH level is elevated.
2. Suppressed TSH levels occur in thyrotoxicosis, thyroid autonomy, nodular goitre, past history of thyrotoxicosis.
3. After thyroid surgery of radioiodine therapy, TSH may be elevated despite clinical euthyroidism. The elevated TSH level is a warning that thyroid hormone concentration is sub-optimal. Such patients require either:
• Thyroxine replacement therapy, or
• Regular follow up to detect the development of clinical thyroid deficiency.
4. TRH-stimulation tests.
T4 total: nmol/L
TSH: 0.40 – 4.00 mU/L
Automated method introduced 22 August 2014
Primary thyroid function screen (comprises T4 and TBG [thyroid binding globulin], thus allowing calculation of the FTI).
Serum T3 and/orTSH routinely initiated if screening result abnormal. Patient sample sufficient for full thyroid function testing.
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