What Causes a Low TSH Level If You Have a Normal Free T4 Level?

Subclinical hyperthyroidism causes a low thyroid stimulating hormone level and a normal free thyroxine, or T4, level, according to WebMD. The cause of subclinical hyperthyroidism may be endogenous due to the overproduction of thyroid hormones, or exogenous due to the administration of thyroid hormones, explains American Family Physician.

Thyroid stimulating hormone stimulates the thyroid gland to produce T4, explains the American Thyroid Association. A low TSH level may mean the patient has an overactive thyroid gland. Low TSH levels are indicative of hyperthyroidism, and the symptoms include tremors in the hands, weight loss, diarrhea, light sensitivity and puffiness around the eyes, reports Lab Test Online.

However, the patient may have low TSH levels without the symptoms of hyperthyroidism, according to WebMD When a TSH test returns abnormal results, the doctor may order a free T4 test to further understand what is wrong with the thyroid, according to Endocrine Web. If the T4 test returns normal results, it means the patient suffers from mild, or subclinical, hyperthyroidism, reports Lab Tests Online.

Subclinical hyperthyroidism is a very mild form of hyperthyroidism, according to WebMD. Patients usually have no symptoms, though some may develop bone and heart complications associated with hyperthyroidism. Endogenous causes of subclinical hyperthyroidism include Grave's disease and multinodular goiter, reports American Family Physician.

Patients may also have subclinical hyperthyroidism if they intentionally take thyroid hormones to treat malignant thyroid diseases, according to American Family Physician. Patients who have hypothyroidism may suffer from subclinical hyperthyroidism due to excessive hormone therapy.