From the Guidelines
A low Thyroid-Stimulating Hormone (TSH) and low normal free Thyroxine (T4) indicate subclinical hyperthyroidism.
Definition of Subclinical Hyperthyroidism
Subclinical hyperthyroidism is defined as a serum TSH concentration below the statistically defined lower limit of the reference range when serum FT4 and T3 concentrations are within their reference ranges 1.
Causes of Low TSH and Normal FT4
Other causes of a low serum TSH with normal concentrations of FT4 include:
- Delayed recovery of the pituitary TSH-producing cells during or after therapy for hyperthyroidism
- Normal pregnancy
- Various nonthyroidal illnesses (euthyroid sick syndrome)
- Administration of dopamine, glucocorticoids, and possibly dobutamine 1
Differentiation from Other Conditions
When serum FT4 is in the normal range, it is almost invariably in the lower part of the range in those with nonthyroidal illness, in contrast to the high normal FT4 concentration of typical subclinical hyperthyroidism 1.
Clinical Significance
Subclinical hyperthyroidism may result from endogenous overproduction of thyroid hormone or intended, or inadvertent, overadministration of thyroid hormone 1.
Diagnosis and Management
The serum TSH test is the primary screening test for thyroid dysfunction, and multiple tests should be done over a 3- to 6-month interval to confirm or rule out abnormal findings 1. Follow-up testing of serum T4 levels in persons with persistently abnormal TSH levels can differentiate between subclinical (normal T4 levels) and “overt” (abnormal T4 levels) thyroid dysfunction 1.