Deep tendon reflexes slow in hypothyroidism mechanisms

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hypothyroidism deep tendon reflex relaxation mechanism

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deep tendon reflex arc mechanism diagram

This pathophysiology diagram illustrates the 'remote preconditioning reflex arc,' a complex neural and humoral pathway involved in remote ischemic preconditioning (RIPre). The diagram depicts a stimulus applied to an arm via a blood pressure cuff, which triggers afferent somatic nerves that project to the brain. Efferent signals are shown descending from the central nervous system via the vagus nerve (n. vagus) and sympathetic nerves to the heart and other visceral organs, specifically the intestine. Key cardiovascular features include the involvement of intrinsic cardiac ganglia and a potential rise in acetylcholine (ACh) concentration in the myocardium. The diagram highlights several scientific uncertainties using question marks, specifically regarding the role of sympathetic nerves, whether abdominal vagal fibers are afferent or efferent, and the origin of humoral factors like GLP-1 and nitrite. Humoral pathways are shown as dashed lines connecting systemic factors to the brain and heart, indicating a parallel mechanism of cardioprotection alongside the primary neural reflex arc.

This clinical procedural photograph demonstrates the technique for eliciting a deep tendon reflex in the upper extremity, specifically targeting the deltoid tendon. A healthcare provider is shown using a Babinski-style reflex hammer, characterized by a slender, light-colored handle and a circular, disk-shaped head with a dark rubberized peripheral ring. The examiner's hand holds the handle between the thumb and fingers to provide a controlled, swinging motion. The patient is positioned supine, with the shoulder and upper arm exposed. The reflex hammer's head is shown at the point of impact on the lateral aspect of the upper arm, near the insertion of the deltoid muscle. This procedure is fundamental in a neurological examination to assess the integrity of the C5 and C6 spinal nerve roots and the axillary nerve. The image serves as an educational tool for medical students and clinicians to visualize proper tool handling and anatomical targeting for upper limb reflex testing.

This historical anatomical illustration depicts René Descartes' early 17th-century mechanistic theory of the reflex arc. The diagram shows a human figure kneeling by a fire, illustrating the physiological response to a noxious stimulus. Key elements are labeled to represent a functional pathway: (a) indicates the external stimulus (fire); (b) represents the peripheral sensory site on the skin of the foot; (c) shows a 'fine thread' or nerve fiber extending from the foot, up the leg and back, to the head. The path includes nodes representing the spinal column or ganglia. Within the brain, (d) and (e) represent mechanical valves that govern the release of 'animal spirits' from a central cavity (f). This anatomical diagram serves as a foundational concept in the history of neuroscience, demonstrating an early understanding of the stimulus-response pathway, where sensory input travels to the central nervous system to trigger a motor reflex. It is a critical visual resource for teaching the evolution of neurophysiology and the history of medicine.

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Slow ("Hung-Up") Deep Tendon Reflexes in Hypothyroidism

The prolonged relaxation phase of DTRs - most dramatically seen at the Achilles tendon - is one of the most reliable bedside signs of hypothyroidism. The phenomenon is termed pseudomyotonia (because it mimics myotonia clinically but lacks the EMG signature of true myotonia). The mechanisms operate at the level of muscle biochemistry and calcium handling, not at the nerve or neuromuscular junction.

1. Reduced Myosin ATPase Activity (Slowed Contraction Phase)

Thyroid hormones directly regulate gene expression of myosin heavy chain (MHC) isoforms:

Thyroid hormone upregulates fast MHC isoforms (MHC-I, MHC-II) and increases myosin ATPase activity.
In hypothyroidism, myosin ATPase activity is decreased. Since the speed of muscle contraction is proportional to the rate of ATP hydrolysis by myosin cross-bridges, both the contraction phase and the relaxation phase are prolonged.

"The speed of the contractile process is related to the quantity of myosin adenosine triphosphate (ATPase), which is increased in hyperthyroid muscle and decreased in hypothyroid muscle."

Adams and Victor's Principles of Neurology, 12th Ed.

2. Impaired Calcium Reuptake by the Sarcoplasmic Reticulum (Key Mechanism of Slowed Relaxation)

This is the dominant mechanism for the delayed relaxation phase specifically:

Muscle relaxation requires rapid re-uptake of cytosolic Ca²⁺ back into the sarcoplasmic reticulum (SR) by the SERCA pump (sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase).
Thyroid hormone stimulates SERCA pump expression and activity. In hypothyroidism, SERCA activity is reduced, so Ca²⁺ removal from the cytoplasm is slow.
As long as cytoplasmic [Ca²⁺] remains elevated, actin-myosin cross-bridges remain engaged and the muscle cannot fully relax.

"The speed of relaxation depends on the rate of release and reaccumulation of calcium in the endoplasmic reticulum. This is slowed in hypothyroidism and increased in hyperthyroidism."

Adams and Victor's Principles of Neurology, 12th Ed.

"The basis of this disorder [pseudomyotonia] appears to be slowness in the reaccumulation of calcium ions in the endoplasmic reticulum and in the disengagement of actin and myosin filaments."

Adams and Victor's Principles of Neurology, 12th Ed.

3. Slowed Actin-Myosin Cross-Bridge Cycling and Disengagement

Even after Ca²⁺ begins to fall, the rate of cross-bridge detachment is slower when myosin ATPase is deficient. The actin-myosin interaction cycle (attach → power stroke → detach) depends on ATP hydrolysis for the detachment step. With less available ATP turnover, the disengagement of filaments is delayed, prolonging the relaxation phase.

4. Shift in Muscle Fiber Composition

Thyroid hormones promote fast-twitch (Type II) fiber dominance. In hypothyroidism:

There is a shift toward slow-twitch Type I fibers, which have inherently slower contraction-relaxation kinetics.
Biopsy findings in hypothyroid myopathy show altered proportions of fiber types (increase in small fibers, either type 1 or 2) and slight distension of the sarcoplasmic reticulum - Adams & Victor's.
This compounding of slow-fiber physiology on top of impaired Ca²⁺ handling amplifies the clinical finding.

5. Reduced Na⁺/K⁺-ATPase Activity (Membrane-Level Contribution)

Thyroid hormone is a major regulator of Na⁺/K⁺-ATPase expression in all tissues. In hypothyroidism, pump activity is reduced, which:

Alters muscle membrane resting potential
Slows membrane repolarization after action potentials
Contributes to the overall sluggishness of the neuromuscular response, though this is secondary to the Ca²⁺ mechanism.

Clinical Correlates

Feature	Mechanism
Prolonged relaxation of Achilles reflex	Impaired SERCA Ca²⁺ reuptake
Slowed contraction phase	Reduced myosin ATPase
Pseudomyotonia (no true myotonic EMG discharges)	Ca²⁺/cross-bridge mechanism, not ionic channel defect
Myoedema (local mounding on percussion)	Abnormal muscle membrane excitability + slow Ca²⁺ clearance
Elevated CK (up to 10x normal)	Muscle fiber injury from metabolic slowing
Corrects with thyroxine replacement	All mechanisms are reversible

"Features of slow muscle contraction and relaxation occur in 25% of patients [with hypothyroidism]; the relaxation phase of muscle stretch reflexes is characteristically prolonged and best observed at the ankle or biceps brachii reflexes."

Harrison's Principles of Internal Medicine, 22nd Ed.

Reflex Arc: Where Does the Delay Sit?

The neural arc itself (sensory afferents, spinal cord interneurons, alpha motor neurons, efferent motor fibers, neuromuscular junction) is not directly impaired in uncomplicated hypothyroid myopathy. The delay is purely at the level of the muscle fiber relaxation kinetics. This is confirmed by EMG: the compound muscle action potential may be normal, and the myopathic changes are metabolic rather than denervatory. (Note: peripheral neuropathy can also rarely complicate hypothyroidism, which would add a separate conduction delay, but this is a distinct process.)

Sources: Adams and Victor's Principles of Neurology, 12th Ed. (Hypothyroid Myopathy, Pathogenesis, Pseudomyotonia sections) | Harrison's Principles of Internal Medicine, 22nd Ed. (Thyroid Disorders - Endocrine Myopathies)

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