Thyroid (T3/T4)
Also known as: Levothyroxine · Liothyronine · Synthroid · Cytomel · Armour Thyroid · NP Thyroid
Thyroid hormones that regulate metabolic rate, energy production, body temperature, cognitive function, and virtually every organ system in the body.
Overview
Thyroid hormones T4 (thyroxine) and T3 (triiodothyronine) are produced by the thyroid gland and regulate the basal metabolic rate of nearly every cell in the body. T4 is the primary secretory product and serves as a prohormone, converted to the active T3 by deiodinase enzymes in peripheral tissues. Hypothyroidism — insufficient thyroid hormone — is one of the most common endocrine disorders, affecting up to 10% of the population (with subclinical hypothyroidism being even more prevalent). Thyroid optimization is a critical component of health optimization protocols. Even "subclinical" hypothyroidism (TSH 2.5-4.5 mIU/L with normal free T3/T4) can cause fatigue, weight gain, cognitive fog, depression, cold intolerance, hair loss, and elevated cholesterol.
Treatment options include T4-only (levothyroxine/Synthroid), T3-only (liothyronine/Cytomel), combination T4/T3, and desiccated thyroid (Armour/NP Thyroid which contains both T4 and T3 in a ~4:1 ratio). Many optimization practitioners prefer combination therapy or desiccated thyroid, as some patients are poor T4-to-T3 converters.
Mechanism of Action
T3 binds to nuclear thyroid hormone receptors (TRα and TRβ), forming heterodimers with retinoid X receptors (RXR) that bind thyroid response elements (TREs) in DNA. This regulates transcription of genes controlling metabolic rate, thermogenesis (UCP1), mitochondrial biogenesis, protein synthesis, lipid metabolism, and cardiac function. T4 is converted to T3 by type 1 and type 2 deiodinase (DIO1, DIO2) enzymes.
Key Benefits
Potential Side Effects
Common Stacks
This peptide is commonly combined with the following compounds for synergistic effects:
Known Interactions
The following interactions have been documented for Thyroid (T3/T4). Always consult a healthcare professional before combining compounds.
Contraindicated (1)
GC-1 is a thyroid receptor beta agonist. Combining with exogenous thyroid hormone risks thyrotoxicosis — excessive thyroid signaling affecting heart, bones, and metabolism.
Scientific References
Quick Reference
Typical Dose
25-200 mcg/day T4; 5-25 mcg/day T3; 30-120 mg desiccated thyroid
Frequency
Daily, typically on an empty stomach in the morning
Route
Oral tablet/capsule
Half-Life
~7 days (T4); ~1 day (T3)
Cycle Length
Ongoing; TSH, Free T3, Free T4 monitored every 6-12 weeks during titration
FDA Status
FDA approved (Synthroid, Cytomel, Armour Thyroid, others)
Need to calculate dosing?
Use our reconstitution calculator to determine exact syringe measurements.
Open CalculatorThis information is for educational purposes only. Consult a qualified healthcare professional before using any peptide. Dosing information reflects commonly reported protocols and may not be appropriate for everyone.
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