In diagnosing Hashimoto's thyroiditis, a physician should assess symptoms and complaints commonly seen in hypothyroidism, examine the neck, and take a detailed history of family members. Blood tests are extremely useful in diagnosing Hashimoto's thyroiditis. The blood tests look at the thyroid function in general. (With hypothyroidism, a high thyroid stimulating hormone and low thyroid hormone would be expected.)
When hypothyroidism is present, the blood levels of thyroid hormones can be measured directly and are usually decreased. However, in early hypothyroidism, the level of thyroid hormones (T3 and T4) may be normal. Therefore, the main tool for the detection of hyperthyroidism is the measurement of the TSH, the thyroid stimulating hormone. TSH is secreted by the pituitary gland. If a decrease of thyroid hormone occurs, the pituitary gland reacts by producing more TSH and the blood TSH level increases in an attempt to encourage thyroid hormone production. This increase in TSH can actually precede the fall in thyroid hormones by months or years. Thus, the measurement of TSH should be elevated in cases of hypothyroidism.
There is one exception. If the decrease in thyroid hormone is actually due to a defect of the pituitary or hypothalamus, then the levels of TSH are abnormally low. As noted above, this kind of thyroid disease is known as "secondary" or "tertiary" hypothyroidism. A special test, known as the TRH test, can help distinguish if the disease is caused by a defect in the pituitary or the hypothalamus. This test requires an injection of the TRH hormone and is performed by an endocrinologist (hormone specialist).
The blood work mentioned above confirms the diagnosis of hypothyroidism but does not point to an underlying cause. A combination of the patient's clinical history, antibody screening, and a thyroid scan can help diagnose the precise underlying thyroid problem more clearly. If a pituitary or hypothalamic cause is suspected, an MRI of the brain and other studies may be warranted. These investigations should be made on a case by case basis.
The blood tests also usually include an analysis of antibodies (anti-thyroid peroxidase antibodies) to aid in the diagnosis. If the gland is large or there are symptoms of esophageal compressive, an ultrasound may be performed to see if the gland is compressing either the esophagus or the trachea (the food and breathing pipes).
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