THYROLAR-1

Clinical safety rating: caution

Comprehensive clinical and safety monograph for THYROLAR-1 (THYROLAR-1).

How it works

Thyrolar-1 is a combination of levothyroxine (T4) and liothyronine (T3). T4 is converted to the active hormone T3 in peripheral tissues. Both forms bind to thyroid hormone receptors, which regulate gene transcription, influencing metabolism, growth, and development.

What the body does with it

Metabolism	Levothyroxine is metabolized primarily by deiodination in peripheral tissues via iodothyronine deiodinases (D1, D2, D3). Liothyronine is mainly deiodinated by D3. Hepatic metabolism includes conjugation (glucuronidation, sulfation) and biliary excretion.
Excretion	Renal excretion of iodide; after deiodination of T3 and T4, iodine is excreted in urine (∼80%) and feces (∼20%).
Half-life	Levothyroxine (T4): 6–7 days; Liothyronine (T3): 1–2 days. In hyperthyroidism, T4 half-life may be reduced to 3–4 days; in hypothyroidism, prolonged to 9–10 days.
Protein binding	>99% bound primarily to thyroxine-binding globulin (TBG), also transthyretin and albumin.
Volume of Distribution	Levothyroxine: 0.15–0.2 L/kg; Liothyronine: 0.4 L/kg. Reflects extensive tissue distribution and binding.
Bioavailability	Oral: approximately 80–95% for levothyroxine; liothyronine is nearly 100% absorbed.
Onset of Action	Oral: detectable effects within 3–5 days; full clinical effect requires 2–4 weeks repeated dosing.
Duration of Action	Levothyroxine: 3–4 weeks after discontinuation; Liothyronine: 2–3 days after discontinuation.

Classification & Brands

Dosing & administration

Oral: 30-60 mg liothyronine (T3) daily, typically initiated at 15 mg/day and titrated upward based on clinical response. Usual maintenance dose 25-50 mg/day.

Dosage form	TABLET
Renal impairment	No specific dose adjustment required for renal impairment; monitor thyroid function tests as use is not recommended in severe renal failure due to decreased clearance of thyroid hormones.
Liver impairment	Use with caution in hepatic impairment; reduce starting dose to 15 mg/day and titrate slowly; monitor thyroid function and adjust based on response.
Pediatric use	Not recommended for children due to lack of safety and efficacy data; alternative therapies preferred.
Geriatric use	Initiate at 15 mg/day; increase by 5-10 mg/day every 2-4 weeks; monitor cardiovascular status closely due to increased sensitivity to thyroid hormones.

Use during pregnancy

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

Clinical note

Comprehensive clinical and safety monograph for THYROLAR-1 (THYROLAR-1).

Breastfeeding	Compatible with breastfeeding; liothyronine (T3) and levothyroxine (T4) are endogenous hormones, excreted in minimal amounts in breast milk. M/P ratio not clinically relevant; monitor infant thyroid function if high maternal doses used.
Teratogenic Risk	First trimester: No evidence of teratogenicity; maternal hypothyroidism itself may increase risk of fetal loss and neurodevelopmental deficits. Second/third trimester: No direct teratogenic effect; adequate maternal thyroid hormone levels are critical for fetal neurodevelopment.

Warnings & precautions

■ FDA Black Box Warning

Not for use in obesity or weight loss; ineffective and dangerous at high doses, especially with cardiovascular disease.

Side Effect Profile

Serious Effects

Absolute Contraindications

["Untreated adrenal insufficiency","Untreated thyrotoxicosis","Hypersensitivity to any component","Recent myocardial infarction (relative)","Uncontrolled hypertension (relative)"]

Clinical Precautions

Precautions

["Cardiovascular risks with over-treatment (tachycardia, arrhythmia, angina, myocardial infarction)","Adrenal insufficiency crisis if used in untreated adrenal insufficiency","Thyrotoxicosis factitia from excessive doses","Increased bone resorption and osteoporosis risk with long-term TSH suppression","Interactions with anticoagulants, diabetes medications, beta-blockers"]

Clinical Tips & Counseling

Drug interactions

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THYROLAR-1

Clinical safety rating: caution

Comprehensive clinical and safety monograph for THYROLAR-1 (THYROLAR-1).

How it works

What the body does with it

Metabolism	Levothyroxine is metabolized primarily by deiodination in peripheral tissues via iodothyronine deiodinases (D1, D2, D3). Liothyronine is mainly deiodinated by D3. Hepatic metabolism includes conjugation (glucuronidation, sulfation) and biliary excretion.
Excretion	Renal excretion of iodide; after deiodination of T3 and T4, iodine is excreted in urine (∼80%) and feces (∼20%).
Half-life	Levothyroxine (T4): 6–7 days; Liothyronine (T3): 1–2 days. In hyperthyroidism, T4 half-life may be reduced to 3–4 days; in hypothyroidism, prolonged to 9–10 days.
Protein binding	>99% bound primarily to thyroxine-binding globulin (TBG), also transthyretin and albumin.
Volume of Distribution	Levothyroxine: 0.15–0.2 L/kg; Liothyronine: 0.4 L/kg. Reflects extensive tissue distribution and binding.
Bioavailability	Oral: approximately 80–95% for levothyroxine; liothyronine is nearly 100% absorbed.
Onset of Action	Oral: detectable effects within 3–5 days; full clinical effect requires 2–4 weeks repeated dosing.
Duration of Action	Levothyroxine: 3–4 weeks after discontinuation; Liothyronine: 2–3 days after discontinuation.

Classification & Brands

Dosing & administration

Oral: 30-60 mg liothyronine (T3) daily, typically initiated at 15 mg/day and titrated upward based on clinical response. Usual maintenance dose 25-50 mg/day.

Dosage form	TABLET
Renal impairment	No specific dose adjustment required for renal impairment; monitor thyroid function tests as use is not recommended in severe renal failure due to decreased clearance of thyroid hormones.
Liver impairment	Use with caution in hepatic impairment; reduce starting dose to 15 mg/day and titrate slowly; monitor thyroid function and adjust based on response.
Pediatric use	Not recommended for children due to lack of safety and efficacy data; alternative therapies preferred.
Geriatric use	Initiate at 15 mg/day; increase by 5-10 mg/day every 2-4 weeks; monitor cardiovascular status closely due to increased sensitivity to thyroid hormones.

Use during pregnancy

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

Clinical note

Comprehensive clinical and safety monograph for THYROLAR-1 (THYROLAR-1).

Breastfeeding	Compatible with breastfeeding; liothyronine (T3) and levothyroxine (T4) are endogenous hormones, excreted in minimal amounts in breast milk. M/P ratio not clinically relevant; monitor infant thyroid function if high maternal doses used.
Teratogenic Risk	First trimester: No evidence of teratogenicity; maternal hypothyroidism itself may increase risk of fetal loss and neurodevelopmental deficits. Second/third trimester: No direct teratogenic effect; adequate maternal thyroid hormone levels are critical for fetal neurodevelopment.

Warnings & precautions

■ FDA Black Box Warning

Not for use in obesity or weight loss; ineffective and dangerous at high doses, especially with cardiovascular disease.

Side Effect Profile

Serious Effects

Absolute Contraindications

["Untreated adrenal insufficiency","Untreated thyrotoxicosis","Hypersensitivity to any component","Recent myocardial infarction (relative)","Uncontrolled hypertension (relative)"]