UNITHROID
Clinical safety rating: caution
Comprehensive clinical and safety monograph for UNITHROID (UNITHROID).
Synthetic T4 (levothyroxine) is converted to T3, which binds to thyroid hormone receptors to regulate gene transcription, increasing basal metabolic rate.
| Metabolism | Primarily hepatic via deiodination (D1, D2) and conjugation (glucuronidation, sulfation), with minor CYP metabolism. |
| Excretion | Renal (approx. 20-40% as unchanged drug and glucuronide conjugates); fecal (minor, via bile). |
| Half-life | 6-7 days for L-thyroxine (T4) in euthyroid patients; prolonged to 9-10 days in hypothyroidism, shortened to 3-4 days in hyperthyroidism. Clinical context: once-daily dosing achieves steady state in 6-8 weeks. |
| Protein binding | >99% bound to thyroxine-binding globulin (TBG), transthyretin (TTR), and albumin. |
| Volume of Distribution | 0.1-0.2 L/kg (T4); clinical meaning: indicates limited extravascular distribution, consistent with extensive protein binding. |
| Bioavailability | Oral: 50-80% (fasting, without interfering substances). |
| Onset of Action | Oral: 3-5 days for measurable metabolic effects; IV: 6-8 hours for onset of metabolic effects. |
| Duration of Action | Oral: 2-3 weeks after single dose due to long half-life; continuous replacement therapy required for chronic hypothyroidism. |
| Molecular Weight | 776.87 |
Initial adult dose: 25-50 mcg orally once daily; titrate by 12.5-25 mcg every 4-6 weeks based on TSH; typical maintenance: 75-150 mcg orally once daily; maximum dose up to 300 mcg daily in severe hypothyroidism.
| Dosage form | TABLET |
| Renal impairment | No specific dose adjustment required for renal impairment; however, monitor thyroid function tests closely. |
| Liver impairment | No specific dose adjustment required for hepatic impairment (Child-Pugh A, B, C); monitor thyroid function tests closely. |
| Pediatric use | Initial dose: 10-15 mcg/kg orally once daily; maintenance: full replacement dose approximately 3-6 mcg/kg/day in infants, 2-4 mcg/kg/day in children; titrate based on TSH and free T4. |
| Geriatric use | Initiate at 12.5-25 mcg orally once daily; titrate by 12.5 mcg every 4-6 weeks; lower starting doses due to increased sensitivity and risk of cardiac complications. |
| 1st trimester | Levothyroxine crosses placenta minimally; used for maternal hypothyroidism; no known teratogenic risk at therapeutic doses. |
| 2nd trimester | Continue therapy to maintain euthyroidism; increased requirements often needed; monitor TSH. |
| 3rd trimester | Continue therapy; maternal hypothyroidism can affect fetal neurodevelopment; dose adjustment may be needed. |
Clinical note
Comprehensive clinical and safety monograph for UNITHROID (UNITHROID).
| Placental transfer | Minimal transfer; limited passage across placenta due to high protein binding and ionization. |
| Breastfeeding | Levothyroxine is excreted into breast milk in low amounts, not expected to cause adverse effects in nursing infants; compatible with breastfeeding. |
| Lactation Rating |
■ FDA Black Box Warning
Not appropriate for treatment of obesity or weight loss; serious or life-threatening toxicity may occur when used for weight reduction, especially in combination with sympathomimetic amines.
| Common Effects | Injection site reactions pain swelling redness Increased liver enzymes |
| Serious Effects |
Untreated thyrotoxicosisUncorrected adrenal insufficiencyHypersensitivity to levothyroxine or any component
| Precautions | Cardiac toxicity in elderly or patients with cardiovascular disease; monitor thyroid function tests; adjust dose in pregnancy; adrenal insufficiency risk; diabetes management changes. |
| Food/Dietary | Avoid high-fiber foods, soy products, walnuts, and grapefruit juice within 1 hour of UNITHROID administration as they may reduce absorption. Coffee and other caffeine-containing beverages should be avoided at the time of ingestion due to decreased absorption. |
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| L1 - Safe |
| Teratogenic Risk | UNITHROID (levothyroxine) is not associated with an increased risk of congenital malformations when used at therapeutic doses. Hypothyroidism itself, if untreated, poses risks of fetal neurodevelopmental deficits, preterm delivery, and low birth weight. No known teratogenic effects in the first trimester. Second and third trimester risks are primarily related to maternal hypothyroidism rather than direct drug effects. |
| Fetal Monitoring | Monitor maternal TSH and free T4 every 4-6 weeks during pregnancy, with goal TSH <2.5 mIU/L in the first trimester and <3.0 mIU/L in later trimesters. Postpartum, reassess thyroid function within 6 weeks. Fetal monitoring includes ultrasound for growth and heart rate as clinically indicated. |
| Fertility Effects | Untreated hypothyroidism may cause ovulatory dysfunction, menstrual irregularities, and infertility. Correction with levothyroxine restores normal thyroid function and improves fertility. No direct adverse effects on fertility from levothyroxine itself. |
| Clinical Pearls | UNITHROID (levothyroxine sodium) should be taken on an empty stomach, at least 30-60 minutes before breakfast, to maximize absorption. Avoid concurrent administration with calcium carbonate, iron supplements, aluminum-containing antacids, bile acid sequestrants, or proton pump inhibitors. Dose adjustments may be needed during pregnancy, with weight change, or when switching from another levothyroxine brand. Monitor TSH levels 6-8 weeks after dose changes. |
| Patient Advice | Take UNITHROID exactly as prescribed, at the same time each day, preferably in the morning on an empty stomach. · Wait at least 30-60 minutes before eating or drinking anything other than water. · Do not take with other medications or supplements such as calcium, iron, antacids, or cholestyramine. Separate by at least 4 hours if needed. · Do not stop taking UNITHROID without consulting your doctor; do not use it to treat weight loss. · Notify your doctor if you become pregnant or if you experience symptoms of hyperthyroidism (e.g., rapid heart rate, anxiety, weight loss) or hypothyroidism (e.g., fatigue, weight gain, cold intolerance). |