Growth Hormone (Somatropin) vs Thyroid (Levothyroxine / Liothyronine T3/T4)

Mechanism

Recombinant human growth hormone (rhGH/somatropin) is a 191-amino acid protein identical to endogenous GH. It binds the GH receptor (GHR), a type I cytokine receptor, activating the JAK2-STAT5 signaling cascade that drives hepatic IGF-1 production — the primary mediator of GH's anabolic effects. GH directly stimulates lipolysis via hormone-sensitive lipase (HSL) activation and inhibits lipogenesis. It promotes chondrocyte proliferation, collagen synthesis, and linear bone growth. GH also enhances protein synthesis through mTOR pathway activation and improves nitrogen balance. Pulsatile secretion patterns are important — continuous GH exposure preferentially drives IGF-1, while pulsatile release favors direct lipolytic effects.

Standard Dosing

Research indicates 1-2 IU/day (0.33-0.67 mg/day) subcutaneously for anti-aging and body composition. Clinical GHD replacement: 0.2-0.6 mg/day titrated to IGF-1 levels.

Timing

Inject subcutaneously in the morning fasted (mimics physiological pulse) or before bed (mimics nocturnal secretion). Rotate injection sites (abdomen, thigh, deltoid). If using with insulin, separate GH injection by several hours. Fasted-state injection preferred for maximal lipolytic effect.

Cycle Duration

Long-term (6-12+ months) for body composition benefits. Clinical GHD replacement is indefinite. Minimum 3-6 months to assess efficacy.

Side Effects

• Fluid retention and edema (dose-dependent, typically resolves in 2-4 weeks)
• Carpal tunnel syndrome and joint pain
• Insulin resistance and elevated fasting glucose
• Headache
• Numbness and paresthesias
• Potential increased cancer risk with chronic supraphysiological IGF-1 levels
• Gynecomastia
• Acromegalic features with long-term excessive dosing (jaw growth, digit enlargement)

Contraindications

• Active malignancy (GH/IGF-1 promotes cell proliferation)
• Active proliferative or preproliferative diabetic retinopathy
• Acute critical illness (GH increased mortality in ICU patients)
• Active intracranial lesion or tumor
• Prader-Willi syndrome with severe obesity or respiratory impairment
• Closed epiphyses (for linear growth indication only)

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Mechanism

Levothyroxine (T4) is a prohormone converted to the active triiodothyronine (T3) by type 1 and type 2 deiodinase enzymes (DIO1/DIO2) in peripheral tissues. T3 binds nuclear thyroid hormone receptors (TRa and TRb), forming heterodimers with retinoid X receptors (RXR) that bind thyroid response elements (TREs) in DNA, directly modulating transcription of genes controlling basal metabolic rate, thermogenesis, mitochondrial biogenesis (via PGC-1a), cardiac output, and neuronal development. T3 also exerts rapid non-genomic effects on mitochondrial respiration, ion channels, and cell membrane transport.

Standard Dosing

Research indicates Levothyroxine (T4): 25-200 mcg daily based on TSH and free T4 levels. Liothyronine (T3): 5-25 mcg daily, often split into 2-3 doses. Combination T4/T3 ratio typically 4:1 to 3:1 when using both.

Timing

Levothyroxine: Take on empty stomach, 30-60 minutes before breakfast or at bedtime (3+ hours after last meal). Separate from calcium, iron, and antacids by 4 hours. Liothyronine: Split into 2-3 daily doses due to short half-life (2.5 hours for T3 vs. 6-7 days for T4).

Cycle Duration

Ongoing for diagnosed hypothyroidism. Optimization protocols may be shorter-term (3-6 months) with reassessment.

Side Effects

• Tachycardia and palpitations (overdose or too-rapid titration)
• Anxiety and insomnia (particularly with T3)
• Tremor
• Weight loss (excessive dosing)
• Hair loss (temporary during initiation or dose changes)
• Heat intolerance and sweating
• Bone density loss at suppressive doses (TSH <0.1)

Contraindications

• Untreated adrenal insufficiency (correct cortisol before thyroid replacement)
• Acute myocardial infarction
• Thyrotoxicosis
• Known hypersensitivity to levothyroxine or liothyronine

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