Hyperthyroidism
Signs & Symptoms
When the thyroid is producing too much hormone, the patient is said to have an overactive thyroid, or hyperthyroidism ("hyper" means too much). Hyperthyroid patients may experience a variety of symptoms, which may include nervousness, irritability, difficulty sleeping, unexplained weight loss, rapid heart rate (tachycardia), enlarged thyroid (goiter), bulging eyes (exophthalmos), intolerance to heat or fine tremor in fingers. Some hyperthyroidism patients present few symptoms however, and a diagnosis based on clinical evidence is not always selfevident.
The most common cause (greater than 90 percent) of hyperthyroidism is Graves' Disease, an autoimmune disorder in which the body produces antibodies to the TSH receptor (TRAb). These autoantibodies bind to thyroid cells and cause an overproduction of thyroid hormones, without being subject to the normal negative feedback loop.
Other causes of hyperthyroidism include excessive thyroid hormone replacement, toxic multinodular goiter and single autonomously functioning thyroid nodules (toxic adenoma).
Whether a patient is overtly hyperthyroid or subclinical hyperthyroid is discerned on the basis of hormone levels. Overt hyperthyroidism is characterized by low TSH levels, and high T4 and T3. In subclinical hyperthyroidism, TSH values are low, but T4 and T3 levels may appear within the normal range. Such patients have an increased risk of becoming overtly hyperthyroid.
Treatment for hyperthyroidism depends on the severity of the condition and other contributing factors, but may include anti-thyroid drugs, thyroid ablation with I131 treatment, or a surgical thyroidectomy. When the need for thyroid removal or inactivation is necessary, patients will eventually become hypothyroid and require thyroid hormone replacement therapy to compensate for lack of thyroid activity.
During development of hyperthyroidism, serum TSH is the optimal test for diagnosing and assessing all phases of the disease. Follow-up with a Free T4 Estimate (FT4E) test can help quantitate the severity of the disease. A FT4E and/or FT3E test is the optimal approach for monitoring the response to antithyroid therapy. The lag in the pituitary reset of TSH secretion dictates that TSH levels will remain undetectable and diagnostically misleading for the first three months following treatment. Thereafter, as a new thyroid status is achieved, TSH again becomes the optimal test for reflecting both clinical and subclinical thyroid hormone excess or deficiency.
For More Information
- American Thyroid Association
- Thyroid Foundation of Canada
- National Graves' Disease Foundation
- Gland Central
- American Association of Clinical Chemistry (Clinical Chemistry Journal)
- The Endocrine Society
- CLIANet (Clinical Laboratory Improvement Amendments)
- NCCLS (National Committee for Clinical Laboratory Standards)
