Active substance: Levothyroxine Sodium
Active life: ~7 days
Detection time: 2–3 weeks or longer depending on dose and duration

Levothyroxine Sodium is a synthetic form of thyroxine (T4), one of the two main hormones produced by the thyroid gland. Medically, it is prescribed to treat hypothyroidism (underactive thyroid), but also it is sometimes used off-label to enhance fat loss, especially during cutting phases.

Levothyroxine is used for its ability to increase the basal metabolic rate. This helps athletes burn more calories, even at rest, which can accelerate fat loss. It is particularly attractive to those preparing for competitions or aiming for a very lean physique.

While Liothyronine (T3) is considered more potent and direct, some individuals use T4 (Levothyroxine) especially in combination with growth hormone (GH), since GH increases the conversion of T4 into the active hormone T3 in peripheral tissues.

Side effects:
Increased heart rate (tachycardia), heart palpitations and arrhythmias, anxiety, insomnia, irritability, excessive sweating and heat intolerance, muscle loss (catabolism) if not paired with anabolic support, bone density loss over time, suppression of natural thyroid function, which can lead to long-term dependence, thyrotoxicosis (thyroid hormone overdose) in extreme cases.

How to use:
100 mcg/day is often the “standard” dose used, advanced users may go up to 200 mcg/day, but this greatly increases risk of thyrotoxicosis, muscle loss, and heart strain.
Take it at least 30–60 minutes before eating or drinking anything other than water.
Avoid taking it with coffee, milk, calcium, iron, or supplements within that 30–60 minute window, as they impair absorption.