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. |
| Molecular Weight | 776.87 |
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 | Levothyroxine is indicated for treatment of hypothyroidism during pregnancy. It does not cross the placenta significantly. Doses may need to be increased. No known teratogenic effects. |
| 2nd trimester | Continue treatment; monitor thyroid function regularly to maintain euthyroid state. No known fetal risks. |
| 3rd trimester | Continue treatment; monitor thyroid function. No known fetal risks. Adequate maternal thyroid hormone is essential for fetal neurodevelopment. |
Clinical note
Comprehensive clinical and safety monograph for THYRO-TABS (THYRO-TABS).
| Placental transfer | Minimal placental transfer; levothyroxine crosses the placenta poorly due to its high protein binding and large molecular size. The placenta also contains deiodinases that regulate transfer. Endogenous thyroid hormone transfer is limited; fetal thyroid hormone production begins around 12 weeks. |
| Breastfeeding | Levothyroxine is excreted into breast milk in low amounts, unlikely to affect the infant. It is considered compatible with breastfeeding. Monitor infant for signs of thyroid suppression if maternal doses are high. |
■ 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 |
Uncorrected adrenal insufficiencyThyrotoxicosis (untreated)Acute myocardial infarctionHypersensitivity to levothyroxine or any inactive ingredients
| 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 |
| Food/Dietary | Take on an empty stomach with water. Avoid taking with high-fiber foods, walnuts, soybean flour, cottonseed meal, or calcium-fortified juices as they may reduce absorption. Separate administration from iron, calcium supplements, antacids, and sucralfate by at least 4 hours. |
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| Lactation Rating | L1 - Safe |
| 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 | 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. |
| Clinical Pearls | THYRO-TABS (thyroid USP, desiccated) is a natural thyroid hormone replacement derived from porcine thyroid glands. It contains both T4 (levothyroxine) and T3 (liothyronine) in a fixed ratio. Monitor thyroid-stimulating hormone (TSH) levels 6-8 weeks after dose adjustment. Use with caution in patients with cardiovascular disease; start at low doses and titrate slowly. Absorption is affected by food, iron, calcium, and fiber; administer on an empty stomach 30-60 minutes before breakfast. |
| Patient Advice | Take THYRO-TABS exactly as prescribed, usually once daily on an empty stomach 30-60 minutes before breakfast with a full glass of water. · Do not stop taking this medication abruptly without consulting your doctor. Dosage adjustments are based on blood tests. · Inform your doctor about all other medications you take, including over-the-counter drugs and supplements, especially iron, calcium, antacids, and fiber supplements. · Report symptoms of hyperthyroidism (rapid heartbeat, chest pain, nervousness, excessive sweating, heat intolerance) or hypothyroidism (fatigue, weight gain, cold intolerance, constipation, depression). · Consistency in taking the medication at the same time each day is important for stable hormone levels. · Store at room temperature away from moisture and heat. Keep out of reach of children. |