The thyroid gland has a central function in human metabolism. It produces the hormones T3 (triiodothyronine), T4 (thyroxine) and calcitonin. While T3 and T4 are involved in numerous processes in energy metabolism, calcitonin plays an important role in calcium metabolism and bone formation. If thyroid disease is suspected, a blood test is usually performed to determine the various thyroid levels. We explain what the individual values mean and what causes may lie behind a deviation from the normal range.
Thyroid values: diagnosis of thyroid disorders.
If a patient’s symptoms and history indicate hyperthyroidism or hypothyroidism, the concentration of the hormones T3, T4, and TSH (thyroid-stimulating hormone, thyrotropin) in the blood provides information about thyroid function. If these values are within the normal range, a thyroid dysfunction is ruled out. If an inflammation of the thyroid (thyroiditis) or an autoimmune disease such as Graves’ disease is suspected, the blood is tested for the presence of specific antibodies against components of the thyroid gland. The concentration of the hormone calcitonin in the blood, on the other hand, is usually measured only to rule out a specific type of thyroid cancer. Understanding laboratory values: A check of the most important abbreviations
Normal range of thyroid values
In the following table, we have compiled an overview of the normal ranges of the various thyroid values for you. However, this overview serves only as an approximate guide, since normal values depend on the patient’s age and sex as well as on the examining laboratory.
| Value | Normal range |
|---|---|
| TSH without stimulation (basal) | 0.3-4.0 mU/l |
| TSH 30 minutes after TRH stimulation (TRH test). | Increase of 2-25 mU/l compared with basal TSH. |
| free T3 (fT3) | 1.7-3.7 ng/l |
| free T4 (fT4) | 7-15 ng/l |
| Antibodies against thyroperoxidase (TPO-AK, anti-TPO). | <80 U/ml |
| Antibodies to thyroglobulin (TAK, anti-TG). | Females: < 100 IU/ml Men: < 60 IU/ml |
| Antibodies against TSH receptor (TRAK, anti-TSH-R). | <9 U/l |
| Calcitonin (HCT) | Women: < 5.0 ng/l Men: < 8.4 ng/l |
| Thyroglobulin (TG) | in healthy thyroid patients: < 75 µg/ml after thyroidectomy: < 3 µg/ml |
Deviations in thyroid levels
If your thyroid levels deviate from the normal range, there may be several reasons. There is not always a serious disease behind a thyroid value that is too high or too low, because blood values can also be subject to daily or seasonal fluctuations. We have summarized for you the meaning and possible causes of increased or decreased thyroid levels.
Thyroid hormones: hyperthyroidism or hypothyroidism?
If hypothyroidism or hyperthyroidism is suspected, basal TSH and free hormones T3 and T4 (fT3, fT4) are usually determined by a blood test. Free means that the hormones are not present in the blood bound to a transporter. The general rule for thyroid levels of fT3 and fT4 is:
- Values too high: hyperthyroidism (hyperthyroidism).
- Values too low: hypothyroidism (hypothyroidism).
In conjunction with the basal TSH value, the doctor can draw conclusions about the cause of thyroid disease. Here, the distinction between a primary and a secondary dysfunction is particularly important. Primary means that the disorder is located in the thyroid gland itself.
Basal TSH: indication of the cause of the dysfunction.
The concentration of thyroid hormones alone does not indicate the cause of thyroid dysfunction. Therefore, basal TSH is also determined. Basal means that TSH production has not been stimulated by administration of TRH in a TRH test. The resulting constellation of thyroid values fT3, fT4 and basal TSH can be interpreted by the physician and assigned to a disease. In this context, a deviation of the basal TSH value may have the following causes:
- Value too high: primary hypothyroidism, secondary hyperthyroidism, medications (MCP drops, certain agents for epilepsy and for mental illness).
- Value too low: primary hyperthyroidism, secondary hypothyroidism, drugs (heparin, cortisone, opiates, drugs for Parkinson’s disease).
Causes of primary hyperthyroidism.
Causes of primary hyperthyroidism may include:
- Graves’ disease
- Thyroiditis in the early stages
- Thyroid autonomy
Causes of primary hypothyroidism.
Possible causes of primary hypothyroidism include:
- Iodine deficiency
- Iodine utilization disorder
- Advanced thyroiditis
- Surgical removal of the thyroid gland
Causes of secondary dysfunction of the thyroid gland.
In a secondary dysfunction, on the other hand, the dysfunction is located in a higher-level organ of the hormonal regulatory circuit, i.e. in the hypothalamus or – more frequently – in the pituitary gland. Secondary hypothyroidism is usually caused by hypopituitarism. In this case, too little TSH is produced by the pituitary gland and the thyroid gland is not sufficiently stimulated to produce T3 and T4. In contrast, secondary hyperthyroidism can be caused by a TSH-producing tumor of the pituitary gland – although this is relatively rare.
TRH testing: safety in borderline results.
If the values of TSH, fT3, and fT4 are in a “gray zone,” i.e., deviate only slightly from the normal range, a TRH test can be performed to confirm the diagnosis. In this test, after the basal TSH has been determined, the hypothalamic hormone TRH is injected or administered as a nasal spray to stimulate the production of TSH in the pituitary gland. After 30 minutes, a blood sample is taken again and the increase in TSH is measured. In healthy individuals, TSH should increase by two to 25 mU/l. Deviations from this range confirm the presence of pituitary or thyroid dysfunction:
- Decreased TSH rise: primary hyperthyroidism (when fT3 and fT4 are elevated), hypopituitarism (when fT3 and fT4 are decreased).
- Excessive TSH increase: primary hypothyroidism.
However, various medications and diseases such as Cushing’s syndrome (excessive production of the hormone cortisone) can also affect the rise in TSH. If the result of the TRH test is within the normal range, thyroid dysfunction is unlikely.
Thyroid antibodies in autoimmune diseases.
To rule out autoimmune disease as a cause of hypothyroidism or hyperthyroidism, the blood can be tested for specific antibodies to components of the thyroid gland:
- Thyroperoxidase antibodies (TPO-AK, anti-TPO): thyroperoxidase is an enzyme that has an important function in the formation of thyroid hormones. Antibodies against thyroperoxidase are usually found in the blood in Hashimoto’s thyroiditis and other forms of thyroiditis, as well as Graves’ disease. However, these antibodies can also be detected in five percent of healthy individuals. A positive anti-TPO result alone is therefore not proof of an autoimmune disease.
- Thyroglobulin antibodies (TAK, TG-AK, anti-TG): thyroglobulin is a storage protein for thyroid hormones. Antibodies against thyroglobulin can be detected in Hashimoto’s thyroiditis, other forms of thyroiditis, Graves’ disease, thyroid cancer and in five percent of healthy individuals.
- TSH receptor antibodies (TRAK, anti-TSH-R): the TSH receptor is the “docking site” for the hormone TSH on the thyroid gland. Antibodies against the TSH receptor are typically present in Graves’ disease. Only in a few cases can they be detected in other thyroid diseases. In healthy individuals, the TRAK value is negative.
Calcitonin
The hormone calcitonin is produced in the so-called C cells of the thyroid gland. It plays an important role in calcium metabolism and promotes bone formation. The calcitonin level in the blood is determined if a form of thyroid cancer (C-cell carcinoma, medullary thyroid carcinoma) is suspected. Abnormal calcitonin levels may be due to the following:
- Value too high: C-cell carcinoma, kidney weakness, hypergastrinemia (increased production of the hormone gastrin in the stomach), birth control pills.
- Value too low: too low calcium level in the blood.
Thyroglobulin
In addition to detecting antibodies to thyroglobulin, the concentration of the protein itself in the blood can also be determined. Thyroglobulin levels are elevated in Graves’ disease, benign enlargement of the thyroid gland (euthyroid goiter), and inflammation of the thyroid gland. The thyroglobulin level has an important significance in control examinations after surgical removal of the thyroid gland due to thyroid cancer. If elevated thyroglobulin levels are detected after thyroidectomy, this may indicate recurrence of the cancer.