Thyroid Hormone Receptors (THR): Structure, Function, and Clinical Significance
Thyroid hormone receptors (THRs) are nuclear receptor proteins that mediate the cellular effects of thyroid hormones, with two main genes—THRA and THRB—encoding distinct isoforms (TRα1, TRα2, TRβ1, TRβ2) that have non-overlapping tissue distributions and clinical consequences when mutated 1, 2.
Molecular Structure and Mechanism
THRs function as ligand-dependent transcription factors within the nuclear receptor superfamily. Structurally, they contain three key domains 1:
- N-terminal domain: Variable region
- DNA-binding domain: Highly conserved across isoforms
- Ligand-binding domain: Mediates interaction with thyroid hormones (T3) and transcriptional coregulators
The active thyroid hormone T3 binds to nuclear THR proteins, which then function as transcription factors regulating gene expression 3. THRs predominantly operate as heterodimers with retinoid X receptor (RXR), binding to thyroid hormone response elements in target genes 1.
Isoform-Specific Functions and Tissue Distribution
THRβ (TRβ1 and TRβ2)
THRβ isoforms are predominantly expressed in:
- Liver (primary site for metabolic effects)
- Pituitary gland (central feedback regulation)
- Cochlea and retina (sensory function)
- Cerebellum (development)
THRβ mediates hepatic lipid metabolism by stimulating hepatic lipophagy, mitochondrial biogenesis, and inhibiting hepatic lipogenesis 4. The β1 receptor selectivity is exploited therapeutically—resmetirom, a liver-directed THRβ agonist, received FDA accelerated approval in March 2024 for non-cirrhotic MASH with significant fibrosis 4.
THRα (TRα1)
TRα1 is predominantly expressed in:
- Heart and cardiovascular system
- Skeletal muscle
- Bone
- Gastrointestinal tract
- Central nervous system
The cardiovascular system shows profound effects from TRα activation, including increased tissue thermogenesis, basal metabolic rate, and reduced systemic vascular resistance 3.
Clinical Relevance: Resistance to Thyroid Hormone Syndromes
RTHβ (Resistance to Thyroid Hormone Beta)
Mutations in THRB cause RTHβ, characterized by 2, 5:
- Elevated free T4 and T3 with normal or elevated TSH (impaired central feedback)
- Goiter
- Sinus tachycardia
- Attention deficit hyperactivity disorder
- Variable tissue hyposensitivity reflecting dysbalance between resistant β-expressing tissues and overstimulated α-expressing tissues
The hallmark is that the hypothalamic-pituitary-thyroid axis is significantly affected because the pituitary expresses predominantly TRβ2 2.
RTHα (Resistance to Thyroid Hormone Alpha)
Mutations in THRA cause RTHα with a distinctly different phenotype 2, 5, 6:
- Mental retardation of variable degree
- Short stature with decreased subischial leg length
- Chronic constipation and bradycardia
- Minimally affected hypothalamic-pituitary-thyroid axis (normal or near-normal TSH)
- Subnormal free T4 with normal TSH and total T3
The stark contrast between RTHα and RTHβ phenotypes demonstrates that THRA and THRB have non-overlapping functions in human physiology 2, 7. This was initially unexpected—mouse studies introducing identical mutations into Thra versus Thrb genes produced distinct phenotypes, suggesting THRA mutations would not be lethal, which proved correct when human cases were identified 2.
Therapeutic Implications
Selective THRβ Agonists
The development of liver-selective THRβ agonists represents a major therapeutic advance 4, 8:
Resmetirom (FDA-approved 2024):
- High selectivity for β1 receptor
- Indicated for non-cirrhotic MASH with fibrosis stage 2-3
- Demonstrated histological efficacy: resolution of steatohepatitis and fibrosis improvement
- Reduced liver enzymes and serum lipids
- Neutral effects on glycemic control and body weight
- Dosing: 80 mg daily for body weight <100 kg, 100 mg for ≥100 kg
Critical caveat: Resmetirom cannot be recommended for cirrhotic-stage MASH due to lack of data 4. Long-term safety, sustainability of histological benefits, and liver-related outcomes remain unknown 4.
Treatment Challenges in THR Disorders
For RTHα patients, levothyroxine replacement therapy shows limited clinical improvement 6. Supraphysiological doses may refine lipid profiles but cause thyrotoxicosis symptoms, reflecting the complex tissue-specific resistance patterns 6.
Diagnostic Considerations
When evaluating suspected THR syndromes 2, 5:
For RTHβ: Look for elevated free T4/T3 with nonsuppressed TSH, goiter, tachycardia, and ADHD symptoms. Genetic testing of THRB confirms diagnosis.
For RTHα: Look for growth abnormalities (short stature, decreased leg length), constipation, bradycardia, and developmental delays with subnormal free T4 but normal TSH. Genetic testing of THRA is diagnostic.
The phenotypic distinction is critical: RTHβ affects central feedback (elevated TSH), while RTHα spares the pituitary axis but profoundly affects growth and development 2.