The basic Blood test for thyroid gland function is TSH, total T4, total T3, Free T4, and Free T3. But the thyroid test cost is high, so that must know when to test for each one, and what’s the clinical need for thyroid function tests and what’s the accurate blood test for thyroid?
What blood test for thyroid function screening
TSH blood test is the one-test screen for patients with non-specific symptoms such as tiredness. A level between 0.4 and 4 mU/L gives 99% exclusion of hyperthyroidism or hypothyroidism.
T4 blood test is added when:
• when there are more specific symptoms of thyroid disease such as tachycardia, atrial fibrillation, weight loss, cold or heat intolerance etc.
• when monitoring thyroid therapy
• suspected hypopituitarism
T3 blood test is added when thyrotoxicosis is suspected.
The combination of clinical and laboratory features will indicate probable status as hyperthyroid, hypothyroid or euthyroid. More about each case later..
What do Thyroid Function Tests mean?
Laboratory blood test for thyroid function is TSH, T3, T4, Free Thyroxine, Free T3, T3 Resin Uptake T3RU, and Thyroid Antibodies, however Lab tests for routine thyroid checkup are TSH and T3.
- Thyroid tests are tested from a blood sample taken from your arm.
- The thyroid gland produces three hormones: thyroxine (T4), triiodothyronine (T3) and calcitonin. T3 and T4 are collectively referred to as thyroid hormone.
- Thyroid hormone controls cellular metabolic activity and is critical to brain development and growth.
- Thyroid hormone secretion is controlled by TSH from the anterior pituitary gland, TSH means thyroid stimulating hormone.
Thyroid-stimulating Hormone (TSH) test
- TSH is the best screening test for thyroid function.
- Increased TSH levels indicate hypothyroidism; decreased TSH levels indicate hyperthyroidism.
- TSH used to monitor thyroid hormone replacement therapy.
Thyroxine Screening test (T4 test)
T4 is the major hormone produced by the thyroid gland. Controls basal metabolic rate.
T4 used to screen for and diagnose hypothyroidism or hyperthyroidism.
Total T4 blood test measures all the T4 forms both protein bound (96–99%) and free (1–4%).
Free Thyroxine blood test (FT4 test)
Free T4 is more accurate than total T4 test, as “Free” T3 and T4 are the metabolically “active” hormones in the plasma.
Total T4 levels can be affected by protein levels in the blood and therefore give false results.
Free or unbound T4 is measured in patients with protein abnormalities to get a more accurate indicator of thyroid
Triiodothyronine Screening test (T3 test)
- Scientifically referred as T3 Radioimmunoassay
- T3 is the major hormone produced by the thyroid gland from the T4 hormone, Used primarily to aid in the diagnosis of hyperthyroidism.
- T3 is 3 to 5 times more active than T4
- Has a more rapid onset of action
- It is also more rapidly degraded in the body.
T3 Resin Uptake test
Other names of the test are T3RU, RT3U, Resin T3 Uptake
Reference range for T3RU is 24–37%
Measures the percentage of thyroid hormone bound to protein and therefore indicates the percentage of binding sites available.
T3 uptake result 37% indicates number of available binding sites is low, which occurs in hyperthyroidism.
T3 uptake result 37% indicates number of available binding sites is high, which occurs in hypothyroidism.
Thyroid Antibodies blood test for thyroid under function and cancer.
They are Antithyroglobulin Antibody and Antimicrosomal Antibody
Confirm the cause of thyroid dysfunction (loss of thyroid function).
Microsomal antibodies are produced in response to microsomes released from damaged thyroid cells.
Calcitonin is a blood test for thyroid cancer
Calcitonin is a hypocalcaemic hormone produced naturally by the parathyroid cells. There are no clinical excess or deficiency states.
Calcitonin reduces circulating calcium levels by increasing calcium’s deposition in bone.
Calcitonin used in the assessment of thyroid medullary cancer, lung cancer, pernicious anemia.
Blood tests for Thyroid diseases symptoms
Thyroid gland is a large ductless gland in the neck that secretes hormones regulating growth and development through the rate of metabolism, thus abnormal secretions of thyroid hormone lead to disturbances in many body biological processes. For such reasons the correct thyroid disease follow up must include these lab tests:
- Liver function tests: Hypothyroidism is a reason of high bilirubin level (Hyperbiliruinaemia) with normal liver enzymes.
- Blood lipogram test: Hypothyroidism is a risk factor of elevated Cholesterol LDL which is a major risk factor for coronary artery disease,
- Serum Triglycerides: TG levels show abnormal results.
- Serum CK test: Hypothyroidism causes Elevations of total serum CK (creatine kinase).
- Glucose Blood test: Hypothyroidism causes Diabetes mellitus (DM) through endocrinopathy.
- Serum Calcium test: Calcium levels are increased in hyperthyroidism. Hypocalcaemia due to hypoparathyroidism which is usually post-thyroidectomy.
- Complete blood count test: An increased neutrophil count may indicate thyroiditis. Decreased neutrophils by Antithyroid drugs, Anemia norocytic or macrocytic
- Xylose Tolerance test: Decreased XTT levels seen in thyroid disease
- Carotene test: hypothyroidism can cause carotenaemia.
- Digoxin test: Hypothyroidism and some drugs raise digoxin levels.
- TSH/T4 used for excluding hyperthyroidism in diagnosis of Anorexia nervosa and bulimia.
I. Tests based on primary function of thyroid, viz. substrate input and hormone synthesis
• Radioiodine “uptake” studies and ‘turnover’ (RAI or RIU) studies
• PBI131 in serum
• T3-suppression test
• TSH-stimulation test
• TRH-stimulation test.
II. Tests measuring blood levels of thyroid hormones
• Serum PBI and BEI
• Circulating T3 and T4 level
• Circulating TSH level
• In vitro resin-uptake of T3
• Plasma tyrosine level
III. Tests based on metabolic effects of thyroid hormones
• Serum cholesterol level
• Serum creatine level
• Serum uric acid
• Serum CK enzyme.
IV. “Scanning” of thyroid gland
V. Immunological tests to detect autoimmune diseases of thyroid gland
• Agar gel diffusion test (precipitation test)
• TRCH test: Tanned red cells haemagglutination test.
• Complement fixation test.
Common Ultrasound Tests:
Assess mass or nodule; follow-up to abnormal thyroid blood tests, history of neck irradiation, or abnormality seen on other tests.
Nuclear Thyroid Scan:
Determine thyroid size, function, and position and evaluate functional status of thyroid nodules.
Evaluate of thyroid and neck masses.
Evaluate of patients with history of head and neck irradiation.
Quantitative thyroid uptake (I-131 uptake).
Next is How to read and understand Thyroid function test results?