Description

Levothyroxine is the synthetic form of thyroxine (T4), a hormone naturally produced by the thyroid gland. Thyroxine is converted peripherally into its biologically active metabolite, L-triiodothyronine (T3). Both T4 and T3 bind to nuclear thyroid receptors, regulating DNA transcription and protein synthesis, with direct effects on cellular metabolism, energy consumption, and the function of numerous organs.

From a chemical point of view, Levothyroxine is a chiral compound in the L form, identical to the endogenous thyroid hormone, ensuring a physiological action comparable to natural thyroxine.

---

Purposes of use

Levothyroxine is primarily indicated for the treatmentof hypothyroidism, a condition characterized by insufficient production of thyroid hormones. Levothyroxine sodium replaces or supplements thyroxine (T4), which the thyroid gland is unable to produce in adequate quantities.

Thyroid hormone deficiency can result from:

• autoimmune diseases (e.g., Hashimoto's thyroiditis)

• surgical removal of the thyroid gland

• radioactive iodine treatments

• damage caused by drugs or radiation

Levothyroxine is also known as L-thyroxine or T4.

---

Effects on metabolism and performance

Optimized metabolism

By restoring physiological levels of thyroxine, Levothyroxine helps normalize basal metabolism, promoting efficient management of energy, body composition, and calorie expenditure.

Increased energy levels

A balanced thyroid function improves vitality, endurance, and the ability to sustain intense workouts.

Muscle recovery

Thyroxine supports protein synthesis and muscle repair processes, making it particularly important during post-workout recovery phases.

Weight management

By regulating metabolism, Levothyroxine helps maintain a stable body weight, which is crucial for both aesthetic and performance goals.

Consistent performance

Stable thyroxine levels ensure continuity in physical and cognitive performance, which is essential for athletes and individuals with high energy requirements.

---

Combined use with growth hormone (HGH)

In individuals taking human growth hormone (HGH), T4 (levothyroxine) supplementation may be necessary. HGH increases the peripheral conversion of thyroxine to T3, with the risk of developing a functional T4 deficiency if not adequately compensated.

---

Dosage and administration

Levothyroxine should be taken on an empty stomach, ideally 30–60 minutes before meals, to maximize intestinal absorption. The tablet should be swallowed with a glass of water.

The dosage depends on:

• age

• body weight

• clinical condition

• individual response

• therapeutic adherence

The maximum dosage can reach 300–400 mcg per day, but such doses are rare and reserved for selected cases under medical supervision.

---

How to use

Generally:

• initial dose: 25–50 mcg per day

• gradual increase of 25–50 mcg every 5–6 days

• average dose: 100–200 mcg per day

The gradual approach allows the thyroid gland and the body to adapt to exogenous thyroxine. Cycles should not exceed 5–7 weeks, with breaks of at least 6 weeks between cycles.

In sports, levothyroxine is sometimes combined with other metabolic agents, although this requires particular caution.

---

Pharmacokinetics

• Oral absorption: 40–80%

• Main site of absorption: jejunum and proximal ileum

• Factors that reduce absorption: food, age, certain diseases, and substances

Half-life of thyroxine:

• 6–7 days in euthyroid subjects

• 9–10 days in hypothyroidism

• 3–4 days in hyperthyroidism

Approximately80% of thyroxine is eliminated via the kidneys, with the remaining 20% eliminated via feces.

---

Main effects

• metabolism regulation

• increased fat burning

• support for anabolic processes

• stimulation of the central nervous system

---

Side effects

Side effects are generally related to excessive dosages and include:

• excessive weight loss

• heat intolerance

• sweating

• anxiety and insomnia

• tremors

• tachycardia

In case of acute overdose, serious events such as fever, hypoglycemia, heart failure, coma, and adrenal crisis may occur.

---

Precautions

Prolonged use of high doses of levothyroxine can suppress endogenous thyroid hormone production. However, when used correctly, thyroid function tends to normalize within a few weeks of discontinuation.

Thyroxine has effects similar to adrenaline, such as increased heart rate and nervousness, which can sometimes be mitigated with beta-blockers under medical supervision.

---

Contraindications

Levothyroxine is contraindicated in cases of:

• hypersensitivity to the active ingredient

• acute myocardial infarction

• Untreated thyrotoxicosis

• uncorrected adrenal insufficiency

---

Drug interactions

Substances that reduce thyroxine absorption:

• calcium and iron (within 4 hours)

• soy (within 3 hours)

• antacids containing aluminum or magnesium

• cholestyramine, colestipol, sucralfate

Other relevant interactions:

• ketamine → tachycardia and hypertension

• tricyclic antidepressants → increased toxicity

• lithium → interference with thyroid metabolism

---

Special warnings

In individuals over 50 years of age or with long-term hypothyroidism, levothyroxine should be introduced gradually. In patients at risk of adrenal insufficiency, corticosteroid coverage is recommended before starting therapy.

Thyroxine may increase the need for insulin or antidiabetic drugs. In addition, excess thyroid hormones may promote bone loss; the dosage should therefore be titrated to the minimum effective level.

---

Storage

• Store at room temperature

• Store in a cool, dry place.

• Protect from direct light

• Keep out of reach of children

• Do not use after the expiration date