No, a TSH of 0.37 with normal T4 indicates subclinical hyperthyroidism, not hypothyroidism
Understanding the Definitions
Subclinical hyperthyroidism is defined as a serum TSH concentration below the lower limit of the reference range (0.45 mIU/L) when serum free T4 and T3 concentrations are within their reference ranges 1. Your TSH of 0.37 mIU/L falls below the established lower limit of 0.45 mIU/L, placing you in the subclinical hyperthyroidism category, not hypothyroidism 1.
In contrast, subclinical hypothyroidism is characterized by an elevated TSH above 4.5 mIU/L with normal free T4 levels 1, 2. This is the opposite of your laboratory findings.
The Normal TSH Reference Range
The reference range for normal serum TSH concentration is defined as 0.45 to 4.5 mIU/L, based on the NHANES III study of disease-free populations 1. The geometric mean TSH in healthy individuals is 1.4 mIU/L 1. Your value of 0.37 mIU/L sits below this established lower threshold.
Clinical Significance of Your Results
Subclinical hyperthyroidism affects approximately 2-3.2% of the population when the lower TSH limit is set at 0.4 mIU/L 1.
While your TSH is suppressed, it is not severely suppressed (which would be <0.1 mIU/L), suggesting mild subclinical hyperthyroidism 1.
Other causes of low TSH with normal free T4 must be excluded, including recent recovery from hyperthyroidism treatment, pregnancy, nonthyroidal illness, or medications such as dopamine or glucocorticoids 1.
Important Considerations
The key distinguishing feature is that when free T4 is in the normal range with low TSH in subclinical hyperthyroidism, the free T4 typically sits in the high-normal range, whereas in nonthyroidal illness with suppressed TSH, the free T4 tends to be in the low-normal range 1. This distinction helps differentiate true subclinical hyperthyroidism from other causes of TSH suppression.
Confirm this finding with repeat testing in 3-6 weeks, as TSH can be transiently affected by acute illness, medications, or physiological factors 1. If persistently low, evaluation for the underlying cause of subclinical hyperthyroidism (such as autonomous thyroid nodules, Graves' disease, or exogenous thyroid hormone) would be warranted 1.