TSH 0.37 uIU/mL at 7 Months Pregnancy: Normal Physiological Finding
A TSH of 0.37 uIU/mL in the third trimester of pregnancy is completely normal and requires no intervention. This value falls within the expected physiological range for pregnancy, when TSH naturally decreases due to hCG-mediated thyroid stimulation 1.
Why This TSH Level is Normal in Pregnancy
Pregnancy causes physiological TSH suppression, particularly in the first trimester but continuing throughout gestation, as human chorionic gonadotropin (hCG) has weak thyroid-stimulating activity that lowers TSH 1.
The general population reference range defines TSH values between 0.45-4.12 mIU/L as normal, with values below 0.1 mU/L considered low 1. Your patient's TSH of 0.37 is only slightly below the lower limit and does not indicate pathological hyperthyroidism.
In disease-free populations, the geometric mean TSH is 1.4 mIU/L, and the reference range represents the 2.5th-97.5th percentile 1. A TSH of 0.37 falls within normal biological variation, especially during pregnancy.
What to Assess
Measure free T4 alongside TSH to distinguish true hyperthyroidism (elevated free T4) from normal pregnancy physiology (normal free T4) 1.
If free T4 is normal or high-normal with TSH 0.37, this confirms physiological pregnancy-related TSH suppression rather than pathological hyperthyroidism 1.
Assess for hyperthyroid symptoms including tachycardia, tremor, heat intolerance, excessive weight loss, or hyperemesis gravidarum, though these are unlikely with TSH 0.37 2.
When TSH 0.37 Would Be Concerning
TSH below 0.1 mIU/L with elevated free T4 would indicate overt hyperthyroidism requiring treatment, as undetectable TSH (<0.04 mIU/L) is associated with thyrotoxicosis in 97% of cases 3.
Subclinical hyperthyroidism (TSH 0.1-0.45 mIU/L with normal free T4) carries intermediate risk of progression, but in pregnancy this range is often physiological 1, 2.
Management Recommendation
No treatment is indicated for TSH 0.37 with normal free T4 in a pregnant woman at 7 months gestation 1.
Recheck TSH and free T4 postpartum at 6-12 weeks, as pregnancy-related TSH suppression should normalize after delivery 1.
If the patient has known thyroid disease or is taking levothyroxine, the dose may need adjustment, but isolated TSH 0.37 in an otherwise healthy pregnant woman requires only reassurance 1.
Critical Pitfalls to Avoid
Do not treat based on a single borderline TSH value without confirming with free T4 measurement and clinical assessment, as 30-60% of mildly abnormal TSH levels normalize spontaneously 1.
Avoid overlooking non-thyroidal causes of TSH suppression, including normal pregnancy physiology, which is the most likely explanation at 7 months gestation 1, 2.
Do not assume hyperthyroidism when TSH is in the 0.3-0.45 mIU/L range with normal free T4 during pregnancy, as this represents normal biological variation 1, 3.