THYROSAFE

Clinical safety rating: caution

Comprehensive clinical and safety monograph for THYROSAFE (THYROSAFE).

How it works

THYROSAFE is a thyroid hormone replacement therapy containing levothyroxine, a synthetic T4 hormone. It exerts its physiological effects by binding to thyroid hormone receptors (TRα and TRβ) in target tissues, modulating gene transcription and cellular metabolism.

What the body does with it

Metabolism	Levothyroxine is metabolized primarily in the liver, kidneys, and peripheral tissues via deiodination by iodothyronine deiodinases (D1, D2, D3). Conjugation (glucuronidation and sulfation) also occurs and is excreted in bile and urine.
Excretion	Primarily renal (60-80%) as unchanged drug; 20-40% as glucuronide conjugates; minimal biliary/fecal (<5%).
Half-life	Terminal elimination half-life is 12-18 hours (mean 15 hours). In hyperthyroidism, clearance may be increased; in renal impairment, half-life prolonged.
Protein binding	90-95% bound, primarily to albumin and transthyretin (thyroxine-binding prealbumin).
Volume of Distribution	0.3-0.5 L/kg, indicating moderate distribution primarily into extracellular fluid; does not readily cross blood-brain barrier.
Bioavailability	Oral: 80-90% absorbed; reduced by food (especially high-fiber, soybean). IV: 100% bioavailable.
Onset of Action	Oral: 2-4 hours for measurable decrease in serum T4; clinical effects in 24-48 hours. IV: 30-60 minutes for T4 reduction; clinical effect within 12 hours.
Duration of Action	Single dose: 24-36 hours. Chronic therapy: sustained effect with continuous dosing; clinical note: requires 3-6 weeks for full remission of hyperthyroid symptoms.

Classification & Brands

Dosing & administration

Initial: 0.075 mg orally once daily; adjust dose every 2-4 weeks based on TSH. Typical maintenance dose: 0.05-0.15 mg orally once daily.

Dosage form	TABLET
Renal impairment	No dose adjustment required for renal impairment.
Liver impairment	No specific guidelines; use with caution in severe hepatic impairment (Child-Pugh class C).
Pediatric use	Weight-based dosing: 0.003-0.005 mg/kg orally once daily; adjust based on TSH and clinical response.
Geriatric use	Start at 0.025 mg orally once daily; increase by 0.025 mg increments every 4-6 weeks.

Use during pregnancy

1st trimester	Consult provider
2nd trimester	Consult provider
3rd trimester	Consult provider

Clinical note

Comprehensive clinical and safety monograph for THYROSAFE (THYROSAFE).

Breastfeeding	Excreted in breast milk; M/P ratio approximately 1.0. Low concentrations are not expected to cause adverse effects in nursing infants, but monitor infant thyroid function. Caution with high maternal doses.
Teratogenic Risk	FDA Pregnancy Category D. First trimester: risk of fetal thyrotoxicosis due to transplacental passage, potential for fetal goiter and hypothyroidism. Second and third trimesters: Continued risk of fetal hypothyroidism and goiter with prolonged use; may cause neonatal hypothyroidism.

Warnings & precautions

■ FDA Black Box Warning

No FDA black box warning. However, note that THYROSAFE should not be used for the treatment of obesity or for weight loss; large doses may cause serious or life-threatening toxicity.

Side Effect Profile

Serious Effects

Absolute Contraindications

["Untreated adrenal insufficiency","Untreated thyrotoxicosis (including hyperthyroidism)","Recent myocardial infarction (relative contraindication)","Hypersensitivity to levothyroxine or any component of the formulation"]

Clinical Precautions

Precautions

["Cardiac toxicity (especially in elderly or patients with cardiovascular disease): start with low dose and titrate slowly.","Adrenal insufficiency: thyroid hormone replacement may increase corticosteroid clearance, potentially precipitating acute adrenal crisis.","Diabetes mellitus: may increase insulin or oral hypoglycemic requirements.","Osteoporosis: long-term suppressive therapy may reduce bone mineral density.","Concomitant use with anticoagulants: enhanced anticoagulant effect.","Pregnancy: dose requirements may increase; monitor thyroid function."]

Clinical Tips & Counseling

Drug interactions

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THYROSAFE

Clinical safety rating: caution

Comprehensive clinical and safety monograph for THYROSAFE (THYROSAFE).

How it works

What the body does with it

Metabolism	Levothyroxine is metabolized primarily in the liver, kidneys, and peripheral tissues via deiodination by iodothyronine deiodinases (D1, D2, D3). Conjugation (glucuronidation and sulfation) also occurs and is excreted in bile and urine.
Excretion	Primarily renal (60-80%) as unchanged drug; 20-40% as glucuronide conjugates; minimal biliary/fecal (<5%).
Half-life	Terminal elimination half-life is 12-18 hours (mean 15 hours). In hyperthyroidism, clearance may be increased; in renal impairment, half-life prolonged.
Protein binding	90-95% bound, primarily to albumin and transthyretin (thyroxine-binding prealbumin).
Volume of Distribution	0.3-0.5 L/kg, indicating moderate distribution primarily into extracellular fluid; does not readily cross blood-brain barrier.
Bioavailability	Oral: 80-90% absorbed; reduced by food (especially high-fiber, soybean). IV: 100% bioavailable.
Onset of Action	Oral: 2-4 hours for measurable decrease in serum T4; clinical effects in 24-48 hours. IV: 30-60 minutes for T4 reduction; clinical effect within 12 hours.
Duration of Action	Single dose: 24-36 hours. Chronic therapy: sustained effect with continuous dosing; clinical note: requires 3-6 weeks for full remission of hyperthyroid symptoms.

Classification & Brands

Dosing & administration

Initial: 0.075 mg orally once daily; adjust dose every 2-4 weeks based on TSH. Typical maintenance dose: 0.05-0.15 mg orally once daily.

Dosage form	TABLET
Renal impairment	No dose adjustment required for renal impairment.
Liver impairment	No specific guidelines; use with caution in severe hepatic impairment (Child-Pugh class C).
Pediatric use	Weight-based dosing: 0.003-0.005 mg/kg orally once daily; adjust based on TSH and clinical response.
Geriatric use	Start at 0.025 mg orally once daily; increase by 0.025 mg increments every 4-6 weeks.

Use during pregnancy

1st trimester	Consult provider
2nd trimester	Consult provider
3rd trimester	Consult provider

Clinical note

Comprehensive clinical and safety monograph for THYROSAFE (THYROSAFE).

Breastfeeding	Excreted in breast milk; M/P ratio approximately 1.0. Low concentrations are not expected to cause adverse effects in nursing infants, but monitor infant thyroid function. Caution with high maternal doses.
Teratogenic Risk	FDA Pregnancy Category D. First trimester: risk of fetal thyrotoxicosis due to transplacental passage, potential for fetal goiter and hypothyroidism. Second and third trimesters: Continued risk of fetal hypothyroidism and goiter with prolonged use; may cause neonatal hypothyroidism.

Warnings & precautions

■ FDA Black Box Warning

No FDA black box warning. However, note that THYROSAFE should not be used for the treatment of obesity or for weight loss; large doses may cause serious or life-threatening toxicity.