TSH Interpreter
Check if your thyroid levels are normal for pregnancy
* Use your laboratory’s pregnancy-specific range when available. A TSH result is considered alongside free T4, thyroid antibodies, symptoms and current treatment. A treatment target can be lower than the diagnostic reference limit. Dose numbers are references for discussion if treatment is prescribed; your clinician selects the dose. Do not start or change levothyroxine from this result alone.
Normal TSH Levels in Pregnancy Chart
Pregnancy changes how TSH is interpreted. Use your laboratory’s pregnancy-specific range when available. This chart uses an upper comparison limit of 4.0 mIU/L when that range is unavailable; its lower limits are broad trimester references.
| Trimester | Reference Range (mIU/L) |
|---|---|
| First Trimester (Weeks 1–12) | 0.1 – 4.0 mIU/L |
| Second Trimester (Weeks 13–26) | 0.2 – 4.0 mIU/L |
| Third Trimester (Weeks 27–40) | 0.3 – 4.0 mIU/L |
Use a pregnancy-specific laboratory range when available. The ATA fallback upper reference limit is about 4.0 mIU/L. A lower target used during treatment is a separate consideration.
Levothyroxine Dosage Chart (Hypothyroidism)
If clinical assessment confirms that treatment is needed, your doctor may prescribe Levothyroxine (synthetic T4) to restore thyroid hormone levels. The starting dose depends on how high your TSH is and your current body weight.
The following dose references apply only when treatment has been prescribed; your clinician chooses the dose and monitoring schedule:
| Condition | TSH Level | Typical Starting Dose |
|---|---|---|
| TSH requiring clinical context | 2.5 – 10 mIU/L | 25 – 50 mcg |
| TSH above 10 — clinical review needed | > 10 mIU/L | Full replacement (~1.6 mcg/kg) |
| Known Thyroid Disease | Already on Meds | Follow your established pregnancy dose plan; contact your prescriber promptly |
How is "Full Replacement" Calculated?
When full replacement is appropriate, clinicians may use a weight-based reference:
For example, a woman weighing 70kg has a calculated full-replacement reference of 112 mcg daily, if that approach is appropriate for her treatment.
Frequently Asked Questions
What causes High TSH in pregnancy?
High TSH can indicate that more thyroid hormone is needed. Your clinician interprets it with free T4, your laboratory range, symptoms and current medication.
Is a TSH of 3.5 dangerous?
A TSH of 3.5 can fall within the pregnancy comparison range when the upper limit is 4.0 mIU/L. Whether treatment is appropriate depends on your laboratory range, thyroid antibodies, free T4 and existing treatment. A treatment target below 2.5 is not the same as a diagnostic threshold.
Methodology: How this works
1. Reference Ranges (The "Traffic Light" Logic)
This calculator compares your input TSH value against trimester-specific cutoffs defined by the American Thyroid Association (ATA) 2017 Guidelines. Unlike standard lab reports which often use a generic "non-pregnant" upper limit of 4.0–5.0 mIU/L, this tool applies the stricter pregnancy thresholds:
- First trimester: use the pregnancy-specific laboratory range when available; this calculator displays the ATA fallback upper comparison limit of about 4.0 mIU/L.
- Second and third trimesters: the displayed fallback upper comparison limit is about 4.0 mIU/L; laboratory-specific limits take priority.
2. Dosage Estimation Algorithm
The dose-reference section remains available above TSH 2.5, independently of whether the result exceeds the displayed comparison range. It shows two reference calculations to discuss if treatment has been prescribed:
- For Mild Elevations (TSH 2.5–10): The tool displays a low-dose reference range; with weight entered, its upper reference is 1.0 mcg/kg. Treatment eligibility and dose selection require clinical assessment.
- For Overt Elevations (TSH > 10): The tool switches to a "Full Replacement" calculation based on body weight, as recommended for patients with little to no thyroid function.
Note: If you enter your weight in pounds (lbs), the calculator automatically divides by 2.20462 to convert to kilograms before running the formula.
Scientific References & Guidelines
- American Thyroid Association (ATA): 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. (Primary source for TSH cutoffs and dosage calculations).
- The Endocrine Society: Management of Thyroid Dysfunction during Pregnancy and Postpartum.
- UpToDate: Hypothyroidism during pregnancy: Clinical manifestations, diagnosis, and treatment.
How the TSH comparison and dose reference work
The pregnancy comparison uses an upper TSH limit of about 4.0 mIU/L when a pregnancy-specific laboratory range is unavailable, following the 2017 ATA guideline, Recommendation 26. The non-pregnancy comparison shown is 0.4–4.5 mIU/L. Lower limits also vary by laboratory. A preconception or treatment target of 2.5 mIU/L is distinct from diagnosing hypothyroidism.
Dose references include a low-dose range and a full-replacement calculation of 1.6 mcg/kg/day. Weight entered in pounds is converted to kilograms. It does not determine whether treatment is indicated. For a TSH between 2.5 and 10, treatment decisions depend on clinical context and thyroid antibodies; see ATA pregnancy guidance. Higher TSH requires prompt clinical review. Dose selection also depends on current medication, age, cardiac history and follow-up blood tests.