Clinician use & disclaimer
Updated with ATA/ACOG guidance and mobile-friendly formatting.
THE INFORMATION IN THE OBRx™ IS INTENDED SOLELY FOR USE BY THE MEDICAL PROFESSION.
It is not intended for lay persons. Medication decisions must be individualized to the patient and local laboratory reference intervals.⚕️ • Review full product labeling and local protocols
Pregnancy: treat to physiologic targets
ATDs cross placenta
Stop ATD + evaluate urgently if fever/sore throat
Hypothyroidism in pregnancy
| Condition | Typical labs | Treatment framing (ATA/ACOG) |
|---|---|---|
| Overt hypothyroidism | TSH above pregnancy reference + low FT4, or TSH >10 mU/L regardless of FT4 |
Treat with LT4 Start ~1–2 mcg/kg/day (often ~100 mcg/day) and titrate to goal. |
| Subclinical hypothyroidism (SCH) | TSH above pregnancy reference + normal FT4 |
TPOAb-positive: LT4 recommended when TSH above pregnancy-specific range; LT4 may be considered when TSH >2.5 but still below upper limit. TPOAb-negative: LT4 may be considered when TSH is above pregnancy-specific range but <10. |
| Isolated hypothyroxinemia | Normal TSH + low FT4 | Do not treat routinely (no proven benefit from LT4 in pregnancy). |
TSH reference intervals (when your lab does not provide trimester-specific ranges)
Preferred: population- and trimester-specific reference ranges from your laboratory.
If unavailable: ATA notes a modest downward shift in early pregnancy and supports an upper TSH reference limit around ~4.0 mU/L in weeks 7–12, with gradual return toward the nonpregnant range later.
Interpret in context (symptoms, antibodies, iodine status, assay). Use local MFM/endocrinology protocols when available.
Levothyroxine (LT4): pregnancy dosing & monitoring
| Scenario | Suggested action | Monitoring / goal |
|---|---|---|
| Already on LT4 and becomes pregnant | Increase LT4 dose by about 20–30% as soon as pregnancy is confirmed (commonly two additional tablets per week of the current daily dose). | Check TSH promptly; then about every 4 weeks until stable (especially first half of pregnancy). Titrate dosage by 12.5 to 25 mcg increments as needed until the patient is euthyroid. Aim for TSH within pregnancy-specific range. |
| New diagnosis: overt hypothyroidism | Start LT4 ~1–2 mcg/kg/day (often ~100 mcg/day; higher if athyreotic). | Titrate to keep TSH in the target range; reassess about every 4 weeks after dose changes. |
| Postpartum | Return toward pre-pregnancy LT4 dose (often the preconception dose). | Recheck TSH about 6 weeks postpartum (or per local protocol). |
Administration pearls (absorption)
- Take LT4 once daily on an empty stomach, ideally 30–60 minutes before breakfast.
- Separate from iron, calcium, prenatal vitamins, and some antacids by at least 4 hours.
- Use consistent brand/generic when possible; recheck TSH after formulation changes.
- Supplied as 0.025, 0.05, 0.075, 0.088, 0.1, 0.112, 0.125, 0.137, 0.15, 0.175, 0.2, and 0.3 mg tablets.
Label information: DailyMed LT4 (example): Levothyroxine sodium
Why avoid desiccated thyroid / T3-containing products in pregnancy?
Major guidelines recommend LT4 as the treatment of choice in pregnancy; T3-containing preparations can complicate biochemical monitoring and fetal thyroid hormone exposure.
Approximate equivalent strengths of various thyroid preparations
Approximate historical equivalence; clinical response and formulations vary. In pregnancy, major guidelines recommend levothyroxine (LT4) as the treatment of choice for hypothyroidism.
| Armour Thyroid (USP) (desiccated thyroid) |
Liotrix (Thyrolar) | Liothyronine (Cytomel) | Levothyroxine (LT4) (Unithroid/Levoxyl/Synthroid, etc.) |
|---|---|---|---|
| 1/4 grain (15 mg) | 1/4 | — | 25 mcg (0.025 mg) |
| 1/2 grain (30 mg) | 1/2 | 12.5 mcg | 50 mcg (0.05 mg) |
| 1 grain (60 mg) | 1 | 25 mcg | 100 mcg (0.1 mg) |
| 1 1/2 grains (90 mg) | 1 1/2 | 37.5 mcg | 150 mcg (0.15 mg) |
| 2 grains (120 mg) | 2 | 50 mcg | 200 mcg (0.2 mg) |
| 3 grains (180 mg) | 3 | 75 mcg | 300 mcg (0.3 mg) |
Source (legacy): United States Pharmacopoeia — Drug Information (historical table). Use current guideline-directed therapy for pregnancy.
Gestational (transient) Hyperthyroidism
Elevated hCG may cause increased FT4 and suppressed or undetectable serum TSH . This occurs commonly with hyperemesis gravidarum. FT4 typically returns to normal by 15 to 18 weeks .Treatment with antithyroid drugs (ATDs) is not indicated. [5]
Hyperthyroidism in pregnancy
Treatment goal:
Free T4 is tested every two weeks and the dosage is adjusted to keep the FT4 in upper third of normal range. Reduce the dosage by 1/2 after FT4 begins to improve. TSH may remain suppressed for weeks/month.
Most common cause: Graves disease
Subclinical hyperthyroidism: do not treat routinely
Gestational transient (hyperemesis/hCG): ATDs usually not indicated
Key maternal/fetal risks if inadequately treated
- Maternal: severe preeclampsia, heart failure, thyroid storm.
- Fetal/neonatal: growth restriction, low birth weight, preterm delivery; neonatal thyroid dysfunction (TRAb-related or medication-related).
Communicate Graves history, TRAb status, and ATD exposure to pediatrics/neonatology at delivery.
Antithyroid drugs (thioamides): PTU & methimazole (Tapazole)
| Medication | Typical dosing (initial / maintenance) | Pregnancy notes |
|---|---|---|
|
Propylthiouracil (PTU) (50 mg tablets) DailyMed label |
Initial (ACOG): 100–600 mg/day PO, usually divided TID (severity-based). Typical adult label start: ~300 mg/day; severe may need 400–900 mg/day initially; maintenance often 100–150 mg/day. |
Preferred 1st trimester Rare but serious hepatotoxicity → consider switching after organogenesis (shared decision-making). |
|
Methimazole (MMI) / Tapazole (5, 10 mg tablets). DailyMed label |
Initial (ACOG): 5–30 mg/day PO, often divided BID initially; can reduce to daily dosing for maintenance. ATA (nonpregnant) FT4-based start guide: see table/tool below. |
Avoid 1st trimester if possible Rare embryopathy risk (e.g., aplasia cutis; choanal/esophageal atresia). Consider MMI after 1st trimester if switching from PTU. |
Baseline labs & “stop now” symptoms (ATA)
- Before starting ATDs, consider baseline CBC with differential and liver profile (bilirubin/transaminases).
- Stop ATD and obtain urgent evaluation if fever, sore throat, or symptoms suggestive of agranulocytosis or hepatic injury.
Methimazole (Tapazole) starting dose by free T4 (ATA hyperthyroidism guideline)
| Free T4 level | Suggested initial methimazole daily dose | Notes |
|---|---|---|
| FT4 = 1.0–1.5 × upper limit of normal (ULN) | 5–10 mg/day | Tailor to symptoms, gland size, and T3 where relevant. |
| FT4 = 1.5–2.0 × ULN | 10–20 mg/day | Use the lowest dose that achieves biochemical control. |
| FT4 = 2.0–3.0 × ULN | 30–40 mg/day | Higher doses increase adverse reaction risk. |
Tapazole dose helper (enter your FT4 and your lab’s ULN)
Enter values to compute the FT4/ULN ratio and view the ATA starting-dose range.
Important: In pregnancy, ATD dosing is individualized—goal is maternal FT4 in the high-normal range or slightly above while minimizing fetal exposure. Consult local endocrinology/MFM protocols.
Switching PTU ↔ MMI after the first trimester (ACOG)
If switching is chosen after the first trimester, a dose ratio of approximately 20:1 (PTU:MMI) is commonly used. Transitions can temporarily worsen biochemical control; coordinate closely with endocrinology/MFM when possible.
Adjunct for symptoms: propranolol
Propranolol may be used short-term for symptomatic palpitations/tremor (avoid long-term high-dose use when possible).
Typical pregnancy dosing: 10–40 mg every 6–8 hours (or 3–4 times daily)
. Adjust dose to keep heart rate at 70 to 90 beats per minute. Beta blockade can be tapered after the free T4 has returned to normal range (~ 3 weeks) [4].
Avoid abrupt cessation of drug.
Thyroid storm
Thyroid storm is rare but life-threatening in pregnancy. Ensure ICU-level care, beta blockade as appropriate, thioamide therapy,
iodine after thioamide, supportive care, and obstetric coordination.
Order set : Thyroid storm order set (Consider updating local protocol link and medication doses per institutional guidance.)
Order set : Thyroid storm order set (Consider updating local protocol link and medication doses per institutional guidance.)
References
Guidelines & key sources
- American Thyroid Association (ATA). 2017 Guidelines for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. PDF
- ACOG Practice Bulletin No. 223 (2020). Thyroid Disease in Pregnancy. PDF
- ATA (Hyperthyroidism). 2016 Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis. PDF (Source of the FT4-based methimazole starting-dose guide.)
- Mestman JH. Hyperthyroidism in pregnancy.Clin Obstet Gynecol. 1997 Mar;40(1):45-64. PMID:9103949
- Stagnaro-Green A, ert al. American Thyroid Association Taskforce on Thyroid Disease During Pregnancy and Postpartum.Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and postpartum. Thyroid. 2011 Oct;21(10):1081-125.PMID .21787128
Drug labels (DailyMed)
Updated 12/21/2025 by Mark Curran, M.D., F.A.C.O.G.