How Sermorelin Affects TSH and Thyroid Function

Sermorelin can affect thyroid function through GH-mediated changes in thyroid hormone metabolism:

1. Type II deiodinase upregulation: GH increases the activity of type II deiodinase (D2), the enzyme that converts T4 (thyroxine, the storage form) to T3 (triiodothyronine, the active form) in peripheral tissues. Increased conversion depletes circulating T4.
2. T4 depletion: With accelerated T4-to-T3 conversion, free T4 levels may decline. If the thyroid gland cannot compensate by increasing T4 production, subclinical hypothyroidism develops (elevated TSH with low-normal or low free T4).
3. Compensatory TSH rise: The hypothalamic-pituitary-thyroid axis detects declining T4 and responds by increasing TSH secretion. This is the hallmark of subclinical hypothyroidism.
4. Pre-existing vulnerability: Users with borderline thyroid function, Hashimoto's thyroiditis, or prior thyroid damage are more susceptible. Sermorelin unmasks marginal thyroid reserve that was previously compensated.
5. Feedback loop: Importantly, untreated hypothyroidism blunts the GH response to sermorelin. The FDA prescribing information states that thyroid function should be assessed before and during treatment because hypothyroidism impairs sermorelin's efficacy.

Incidence: 6.5% of patients in clinical trials developed subclinical hypothyroidism during sermorelin therapy (per Geref prescribing information).

Typical pattern:

• TSH rises from normal range (0.4-4.0 mIU/L) to mildly elevated (4.5-10 mIU/L)
• Free T4 may decline to the lower end of the reference range or below
• Free T3 may initially increase (from accelerated conversion) before normalising
• Overt hypothyroidism (TSH above 10 with low free T4) is uncommon but possible

Timeline: Thyroid changes typically develop gradually over weeks to months of sermorelin use. The onset is slower than the GH/IGF-1 response.

Risk factors for thyroid impact:

• Baseline TSH above 2.5 mIU/L (upper-normal)
• Positive thyroid peroxidase (TPO) antibodies
• History of thyroid disease or thyroid surgery
• Concurrent use of compounds that stress thyroid function (e.g., caloric restriction, DNP)

Baseline: TSH and free T4 before starting sermorelin. If TSH is above 4.0 or free T4 is low-normal, evaluate thyroid function before initiating.

During use:

• TSH and free T4 at 6 weeks, 3 months, then every 6 months
• If symptoms of hypothyroidism develop (fatigue, cold intolerance, weight gain, constipation, dry skin), test immediately regardless of schedule

Action thresholds:

• TSH 4.5-10 mIU/L with normal free T4: subclinical hypothyroidism. Consider thyroid hormone replacement, especially if symptomatic
• TSH above 10 mIU/L or low free T4: overt hypothyroidism. Start levothyroxine and reassess sermorelin

Prevention:

• Screen thyroid function before starting sermorelin
• Address any borderline thyroid issues before initiating GH peptide therapy
• Ensure adequate iodine and selenium intake (both are required for thyroid hormone synthesis and conversion)

If subclinical hypothyroidism develops:

• Low-dose levothyroxine (25-50 mcg/day) typically resolves the TSH elevation
• Thyroid replacement does not negate sermorelin's GH benefits; it actually improves them (hypothyroidism blunts GH response)
• Continue sermorelin if thyroid is adequately replaced

If switching from sermorelin:

• Tesamorelin and ipamorelin have lower thyroid risk (not zero, but not documented at the 6.5% rate of sermorelin)
• Thyroid function typically normalises within weeks of stopping sermorelin