What is the equivalent dosage of Synthroid (levothyroxine) and Cytomel (liothyronine) for 120 milligrams of Armour Thyroid (thyroid hormone replacement medication)?

From the Guidelines

To convert 120 mg of Armour Thyroid to Synthroid (levothyroxine) and Cytomel (liothyronine), the equivalent doses would be approximately 114 mcg of Synthroid and 27 mcg of Cytomel daily, based on the most recent guidelines. When considering the conversion, it's essential to take into account the patient's age, weight, and comorbidities, as these factors can influence the optimal dosage, as suggested by 1. For patients without risk factors, a full replacement dose can be estimated using ideal body weight, approximately 1.6 mcg/kg/d, according to 1. However, for those older than 70 years or frail patients with multiple comorbidities, including cardiac disease, it's recommended to start with a lower dose, such as 25-50 mcg, and titrate up as needed, as indicated by 1. Key considerations for the conversion include:

• Starting with a lower dose of Synthroid, such as 100 mcg, and Cytomel, such as 25 mcg, and adjusting based on thyroid function tests after 6-8 weeks
• Taking Synthroid once daily on an empty stomach and splitting the Cytomel dose into two daily doses
• Monitoring symptoms closely and performing regular thyroid function tests to adjust the dose as necessary
• Being aware that individual responses may vary and adjustments to the dose may be necessary to achieve optimal thyroid hormone levels and symptom control. It's crucial to prioritize the patient's morbidity, mortality, and quality of life when determining the optimal dosage, and to consult the most recent and highest-quality studies, such as 1, to inform clinical decision-making.

From the FDA Drug Label

Twenty-five mcg of liothyronine is equivalent to approximately 1 grain of desiccated thyroid or thyroglobulin and 0. 1 mg of L-thyroxine.

The equivalent dosage of Synthroid (levothyroxine) and Cytomel (liothyronine) for 120 milligrams of Armour Thyroid (thyroid hormone replacement medication) cannot be directly determined from the provided information. However, we can calculate the equivalent dosage as follows:

• 1 grain of desiccated thyroid is equivalent to 25 mcg of liothyronine and 0.1 mg of levothyroxine.
• 120 milligrams of Armour Thyroid is equivalent to 120/0.065 = 1846 grains of desiccated thyroid (assuming 1 grain is approximately 65 mg)
• This is equivalent to 1846 * 25 mcg = 46150 mcg of liothyronine
• This is equivalent to 1846 * 0.1 mg = 184.6 mg of levothyroxine

Key points:

• The dosage of thyroid hormones is determined by the indication and must in every case be individualized according to patient response and laboratory findings 2.
• Liothyronine Sodium Tablets, USP are intended for oral administration; once-a-day dosage is recommended 3. Note that these calculations are approximate and may not reflect the actual equivalent dosage, as the conversion rates may vary depending on the specific medication and individual patient factors.

From the Research

Equivalent Dosage of Synthroid and Cytomel

To determine the equivalent dosage of Synthroid (levothyroxine) and Cytomel (liothyronine) for 120 milligrams of Armour Thyroid, we need to consider the composition of Armour Thyroid and the bioavailability of its components.

• Armour Thyroid is a natural porcine derivative thyroid supplement that contains both T4 and T3 [ 4 ].
• The exact composition of Armour Thyroid can vary, but it is generally estimated to contain approximately 38 mcg of T4 and 9 mcg of T3 per 60 mg tablet [ 5 ].
• Based on this estimate, 120 mg of Armour Thyroid would contain approximately 76 mcg of T4 and 18 mcg of T3.

Equivalent Dosage of Synthroid

The equivalent dosage of Synthroid (levothyroxine) can be estimated based on the T4 content of Armour Thyroid.

• A study published in 1982 found that the bioavailability of T4 from desiccated thyroid tablets (such as Armour Thyroid) was similar to that of synthetic T4 tablets (such as Synthroid) [ 5 ].
• Based on this study, the equivalent dosage of Synthroid for 76 mcg of T4 in Armour Thyroid would be approximately 75-100 mcg of Synthroid.

Equivalent Dosage of Cytomel

The equivalent dosage of Cytomel (liothyronine) can be estimated based on the T3 content of Armour Thyroid.

• A study published in 1982 found that the bioavailability of T3 from desiccated thyroid tablets (such as Armour Thyroid) was similar to that of synthetic T3 tablets (such as Cytomel) [ 5 ].
• Based on this study, the equivalent dosage of Cytomel for 18 mcg of T3 in Armour Thyroid would be approximately 15-20 mcg of Cytomel.

Combination Therapy

It's worth noting that some studies suggest that combination therapy with both T4 and T3 may be beneficial for some patients with hypothyroidism [ 6 , 7 ].

• A study published in 2020 found that reducing the LT4 dose by 25 mcg/day and adding 2.5-7.5 mcg of LT3 once or twice a day was an effective starting point for combination therapy [ 6 ].
• Another study published in 2019 found that the optimal dosage of LT4 and LT3 depended on the patient's residual thyroid function and that a nomogram could be used to estimate the optimal dosage [ 7 ].