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. |
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 | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for UNITHROID (UNITHROID).
| Breastfeeding | Levothyroxine is excreted into breast milk in very low amounts; milk-to-plasma ratio is approximately 0.5. Doses used for maternal hypothyroidism do not affect infant thyroid function or cause adverse effects. Compatible with breastfeeding. |
| 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. |
■ 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 thyrotoxicosis; uncorrected adrenal insufficiency; hypersensitivity to levothyroxine or any excipients.
| Precautions | Cardiac toxicity in elderly or patients with cardiovascular disease; monitor thyroid function tests; adjust dose in pregnancy; adrenal insufficiency risk; diabetes management changes. |
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| 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. |