Can thyroid hormone signaling alter SHBG and testosterone fractions?

The liver is the principal source of circulating sex hormone-binding globulin (SHBG), a glycoprotein that binds sex steroids with high affinity. Thyroid hormone signaling acts directly on hepatic tissue to regulate this process, establishing a critical thyroid-liver-gonadal axis.

Clinical evidence

• Distortion of Testosterone Fractions: Because SHBG binds testosterone with high affinity ($\approx 10^9\text{ L/mol}$), shifts in its concentration dramatically alter the ratio of bound (total) to unbound (free) testosterone.
• Hyperthyroidism / Over-replacement: Excess thyroid hormone signaling (high $\text{T}_3/\text{T}_4$) markedly elevates circulating SHBG. This expands the binding reservoir, raising measured total testosterone while reducing the active percentage of free testosterone. Patients can experience clinical symptoms of androgen deficiency despite having normal or even elevated total testosterone levels.
• Hypothyroidism: Underactive thyroid signaling reduces hepatic SHBG synthesis. While total testosterone levels may appear artificially low, the biologically active free fraction is often relatively preserved, potentially masking or mimicking hypogonadism depending on the assay used.
• Diagnostic Impact: These fluctuations heavily distort calculated and analog free testosterone assays, making gold-standard measurements like equilibrium dialysis critical for patients with concurrent thyroid dysfunction.

Mechanistic explanations

[Thyroid Hormones (T3/T4)] 
       │
       ▼ (Promotes fatty acid beta-oxidation)
[Reduced Palmitate] ──(Disinhibits)──► [HNF4A Expression & Activity]
                                              │
                                              ▼ (Binds proximal promoter)
                                     [Hepatic SHBG Transcription]
                                              │
                                              ▼
                                     [Increased Circulating SHBG]
                                              │
                                              ▼
                                     [Reduced Free Testosterone %]

• Indirect Transcriptional Activation: The human proximal SHBG promoter is TATA-less and lacks a classical, direct thyroid hormone response element (TRE). Instead, thyroid hormones act indirectly.
• HNF4A Upregulation: Thyroid hormones promote fatty acid $\beta$-oxidation in hepatocytes, lowering cellular levels of palmitate (an inhibitor of hepatocyte nuclear factor-4 alpha, or HNF4A). Simultaneously, thyroid signaling directly upregulates HNF4A gene expression.
• Promoter Binding: The resulting accumulation of active HNF4A allows it to bind the FP1 response element on the SHBG promoter, outcompeting transcriptional repressors like COUP-TF and driving robust transcription and secretion of SHBG.

Bottom line

• Thyroid-liver cross-talk directly dictates the relationship between total and free testosterone. Thyroid hormone-induced changes in hepatic HNF4A drive SHBG synthesis, meaning thyroid function must be stabilized and considered to accurately interpret testosterone panels.