Both hypothyroidism and hyperthyroidism significantly reduce heart rate variability. What's surprising is that hyperthyroidism has even larger effects than hypothyroidism.
The Hypothyroidism Effect
A 2022 meta-analysis of 17 studies (11,438 patients) found dramatic HRV reductions in untreated hypothyroid patients[1]:
- RMSSD: SMD = -1.66 (large effect)
- SDNN: SMD = -1.27 (large effect)
- pNN50: SMD = -1.41 (large effect)
- Total Power: SMD = -1.55 (large effect)
- LF/HF ratio: SMD = +1.26 (sympathetic dominance)
All comparisons p < 0.001. Effects increased substantially when TSH exceeded 10 mIU/L.
The Hyperthyroidism Effect (Even Worse)
A parallel meta-analysis of 22 studies (10,811 patients) found even larger effects in hyperthyroid patients[2]:
- SDNN: ES = -6.07 (very large effect)
- HFnu: ES = -3.51 (very large parasympathetic reduction)
- VLF power: ES = -2.65
- Total power: ES = -2.05
- LF/HF ratio: ES = +1.75 (sympathetic dominance)
Critically, most parameters had effect sizes twice as high in overt vs subclinical hyperthyroidism.
Treatment Helps (But May Not Fully Normalize)
A 2024 study followed 110 hypothyroid patients with 24-hour ECG monitoring before and after 3 months of levothyroxine treatment[3]:
- TSH normalized from 9.89 to 3.65 mIU/L (p < 0.0001)
- SDNN improved by 30.38 units
- RMSSD improved by 30.09 units
- HF power increased by 140 units
- LF/HF ratio decreased by 0.96 (restored balance)
However, patients with severe hypothyroidism showed only partial recovery, not complete normalization.
Why This Happens
The thyroid gland and autonomic nervous system share a common control center: the hypothalamus. Thyroid hormones affect autonomic function through multiple pathways:
- Direct effects on autonomic centers
- Altered catecholamine (adrenaline/noradrenaline) production
- Changed cardiac responsiveness to autonomic signals
- Modified beta-adrenergic receptor sensitivity
- Metabolic rate changes affecting overall autonomic tone
The Practical Takeaway
If you have a thyroid disorder and your HRV is low:
- Don't compare yourself to population norms
- Track your personal trends over time
- Expect HRV to improve with proper treatment (but possibly not to "normal" levels)
- Both hypo and hyper conditions affect HRV - hyperthyroidism actually more severely
- Overt disease > subclinical in impact (treatment of subclinical may have less dramatic effects)
HRV may be a useful non-invasive way to track thyroid-related autonomic function, especially if you're monitoring treatment effectiveness.
Sources
1. Borowik A et al. (2022). Heart rate variability in hypothyroid patients: A systematic review and meta-analysis. PLOS ONE. accessibility.link.new-tab (17 studies, n=11,438)
2. Borowik A et al. (2022). Heart Rate Variability in Hyperthyroidism: A Systematic Review and Meta-Analysis. Int J Environ Res Public Health. accessibility.link.new-tab (22 studies, n=10,811)
3. Popławska-Kita A et al. (2024). Hypothyroidism and Heart Rate Variability: Implications for Cardiac Autonomic Regulation. Diagnostics. accessibility.link.new-tab (n=110, 3-month treatment follow-up)