How Growth Hormone Affects Free T3

Growth hormone (GH) has a well-documented effect on thyroid hormone metabolism through modulation of the deiodinase enzyme system:

1. Type 1 deiodinase (D1) upregulation: GH stimulates the expression of type 1 deiodinase, primarily in the liver and kidneys. D1 converts inactive T4 (thyroxine) to active T3 (triiodothyronine) by removing an iodine atom from the outer ring.
2. Increased T4-to-T3 conversion: With enhanced D1 activity, more circulating T4 is converted to T3. This raises free T3 while simultaneously lowering free T4.
3. TSH response: If free T3 rises sufficiently, the HPT axis detects the increase and may mildly suppress TSH to compensate. However, the TSH change is usually subtle because the feedback loop responds primarily to intrapituitary T3 levels.
4. Reverse T3 effects: GH may also influence the ratio of T3 to reverse T3 (rT3), favouring active T3 production. Reverse T3 is an inactive metabolite, so this shift enhances overall thyroid hormone activity.

This interaction has practical importance in bodybuilding: users combining GH with exogenous T3 may develop symptoms of thyroid excess (tachycardia, anxiety, sweating) that they would not experience with either compound alone.

Low-dose GH (1-3 IU/day):

• Free T3 may rise 10-20% above baseline
• Free T4 may decline slightly
• TSH usually remains within normal range
• Clinically insignificant for most users

Moderate doses (4-6 IU/day):

• Free T3 can rise 20-40% above baseline
• Free T4 may drop to the low-normal range
• TSH may be mildly suppressed (0.3-1.0 mIU/L)
• Some users notice increased metabolic rate, warmth, and slight weight loss

High doses (8-15+ IU/day):

• Free T3 may rise 30-50%+ above baseline
• Free T4 can drop below normal range
• The "high T3, low T4" pattern on bloodwork is characteristic of GH use
• Combined with exogenous T3 (common in bodybuilding), total thyroid hormone activity can become excessive

Timeline: The effect on T3 develops gradually over 2-4 weeks as deiodinase expression increases. The T3 elevation persists for as long as GH is administered and resolves within 1-2 weeks after discontinuation.

Baseline: Full thyroid panel (TSH, free T4, free T3) before starting GH.

During use: Check thyroid panel at 4-8 weeks. The classic finding is elevated free T3 with low-normal or low free T4 and mildly suppressed TSH.

When combining GH with T3: Monitor free T3 at 2-3 weeks. The additive effect of GH-enhanced conversion plus exogenous T3 can produce excessively high free T3 levels.

Frequency: Every 3-6 months for stable GH users. More frequently if adjusting doses or combining with T3.

Interpret thyroid panels carefully on GH:

• Low TSH + low free T4 + high free T3 = GH-induced deiodinase upregulation (expected)
• Do not mistake this pattern for hyperthyroidism; it is a pharmacological effect, not thyroid disease
• If TSH is suppressed but free T3 is not elevated, investigate other causes

If free T3 is elevated but asymptomatic:

• No intervention required
• The increased T3 contributes to GH's fat-burning effects and is generally beneficial
• Monitor at regular intervals

If free T3 is elevated with symptoms (tachycardia, anxiety, tremor):

• Consider reducing GH dose
• If taking exogenous T3 concurrently, reduce T3 dose first
• Check resting heart rate; persistent tachycardia above 90-100 bpm warrants dose adjustment

Combining GH with T3 (common in bodybuilding):

• Start T3 at a lower dose (25 mcg/day) when using GH, as GH already increases T3 production
• Some users take T3 specifically to "replace" the T4 that GH diverts to T3, but this rationale is flawed; endogenous T3 is already elevated from GH
• Monitor free T3 closely; the combined effect can produce levels 2-3x the upper limit of normal

If free T4 drops below normal:

• This is expected and usually clinically insignificant when free T3 is adequate
• T4 is a prohormone; as long as active T3 levels are sufficient, low T4 is not concerning
• If both T4 and T3 are low, this suggests a different problem (true hypothyroidism) and warrants investigation

Long-term GH users: Annual comprehensive thyroid panels are recommended. The chronic shift toward T3 dominance is generally benign but should be documented and monitored.