THYRO-TABS
Clinical safety rating: caution
Comprehensive clinical and safety monograph for THYRO-TABS (THYRO-TABS).
THYRO-TABS (levothyroxine) is a synthetic form of thyroxine (T4) that is deiodinated to triiodothyronine (T3) in peripheral tissues, binding to thyroid hormone receptors to regulate gene transcription involved in metabolism, growth, and development.
| Metabolism | Primarily hepatic deiodination via deiodinases to T3; minor glucuronidation and sulfation; some enterohepatic circulation. |
| Excretion | Renal (approx. 40-50% as unchanged drug and metabolites, primarily as glucuronide conjugates), fecal (approx. 20-30% via biliary elimination). Minor amounts excreted as unchanged levothyroxine in urine. |
| Half-life | Terminal elimination half-life of levothyroxine is approximately 6-7 days in euthyroid individuals; prolonged to 9-10 days in hypothyroidism and shortened to 3-4 days in hyperthyroidism. Half-life may be reduced in patients receiving concurrent enzyme-inducing drugs. |
| Protein binding | Levocarnitine? Actually, Thyro-Tabs (levothyroxine) is >99% bound to serum proteins, principally thyroxine-binding globulin (TBG), transthyretin (thyroxine-binding prealbumin, TBPA), and albumin. |
| Volume of Distribution | Apparent volume of distribution of levothyroxine is approximately 0.4-0.5 L/kg; this reflects distribution into body tissues including liver, kidney, and muscle, with a small free fraction in plasma. |
| Bioavailability | Oral: 50-80%, variable due to food, formulation, and gastrointestinal factors; bioavailability is reduced by food (especially fiber, soy, and iron) and by drugs that bind thyroid hormone (e.g., calcium carbonate, bile acid sequestrants). |
| Onset of Action | Oral: Clinical effects appear within 3-5 days; full therapeutic effect may require 2-4 weeks of continuous therapy. |
| Duration of Action | Duration of action after single oral dose: 2-3 weeks for return of euthyroid state; with chronic therapy, effect persists as long as dosing continues. Clinical monitoring of TSH levels recommended at 6-8 week intervals after dose changes. |
Oral, 12.5-25 mcg/day initially, titrated by 12.5-25 mcg every 2-4 weeks based on TSH; typical maintenance dose 50-200 mcg/day.
| Dosage form | TABLET |
| Renal impairment | No dose adjustment required for GFR >=15 mL/min. For GFR <15 mL/min (ESRD), consider reducing starting dose by 25-50% and titrate based on TSH closely. |
| Liver impairment | Child-Pugh Class A and B: no adjustment. Child-Pugh Class C: reduce starting dose by 50% and titrate cautiously. |
| Pediatric use | Initial: 5-10 mcg/kg/day orally; maintenance: 4-6 mcg/kg/day. For congenital hypothyroidism: 10-15 mcg/kg/day starting dose. Adjust based on TSH and T4 levels. |
| Geriatric use | Start with 12.5-25 mcg/day orally; increase by 12.5 mcg every 4-6 weeks. Monitor TSH more frequently; target TSH may be higher in very elderly (e.g., 4-6 mIU/L). |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for THYRO-TABS (THYRO-TABS).
| Breastfeeding | Minimal transfer into breast milk; doses up to 200 mcg/day are considered safe. M/P ratio approximately 0.6. |
| Teratogenic Risk | Pregnancy Category A. No evidence of increased risk of congenital anomalies with replacement doses. Uncontrolled hypothyroidism is associated with higher risk of fetal loss, preterm delivery, and neurodevelopmental deficits. |
| Fetal Monitoring |
■ FDA Black Box Warning
Not appropriate for treatment of obesity or weight loss; use in euthyroid patients may cause serious or life-threatening toxicity, especially if combined with sympathomimetics.
| Serious Effects |
["Untreated thyrotoxicosis (e.g., Graves disease)","Uncorrected adrenal insufficiency","Hypersensitivity to levothyroxine or any ingredient"]
| Precautions | ["Cardiovascular effects: Tachycardia, arrhythmias, hypertension, especially in elderly or patients with underlying heart disease","Thyrotoxic crisis: May precipitate in excessive dosing or rapid dose escalation","Bone density loss: Long-term TSH suppression may lead to decreased bone mineral density","Adrenal insufficiency: In patients with secondary hypothyroidism, corticosteroids should be given before thyroid therapy","Diabetic therapy: May increase blood glucose levels, requiring adjustment of antidiabetic medications"] |
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| Monitor maternal TSH every 4-6 weeks during pregnancy; fetal heart rate and growth ultrasound as clinically indicated. |
| Fertility Effects | Thyroid hormone replacement restores euthyroid state, improving fertility in hypothyroid women. Hyperthyroidism from overtreatment may impair fertility. |