Effects of thyroid hormone on the Cardiovascular System
Positive chronotropic, inotropic and lusitropic effect.
Increases systolic BP and decreased Diastolic BP
Increased Pulse pressure and mean arterial pressure remains somewhat constant
Indirect effects:
Increased heat production and Co2 in tissues- Decreased peripheral vascular resistance-Derceased Diastolic blood pressure
Reflex increase in adrenergic stimulation
Direct effects:
Increased cardiac muscle Myosin heavy chain alpha/beta ratio
sarcoplasmic Ca-ATPase
B-adrenergic signaling
G-protein Increases ryanodine calcium channels
Inhibits expression on Na-Ca antiporter
Increased ventricular contractility and function
Effects of thyroid hormone on the respiratory system
Thyroid hormones stimulate oxygen utilization
Increases resting respiratory rate, minute ventilation, and the ventilatory response to hypercapnia and hypoxia
Increase hematocrit secondary to EPO secretion
Increase dissociation of O2 from hemoglobin by increasing red cell 2,3-diphosphoglycerate (DPG).
Effects of thyroid hormone on the Nervous System
•Growth of cerebral and cerebellar cortices.
•Proliferation of axons and branching of dendrites, synaptogenesis, myelination, and cell migration during brain development.
•Enhances wakefulness, alertness, and responsiveness to various stimuli.
•The speed and amplitude of peripheral nerve reflexes are increased.
•Memory, learning, and intellectual capacities.
Too much induces anxiety and nervousness, constant tiredness & difficulty sleeping due to the excitable effect of thyroid hormone on synapse
Too little thyroid hormone- the individual tends to feel mentally sluggish & extremely somnolence
Increased reactivity of the neuronal synapses in the areas of the spinal cord that control muscle
Effects of thyroid hormones on the autonomic nervous system
Thyroid hormones are synergistic with catecholamines in increasing the metabolic rate, heat production, heart rate, and excitation of the central nervous system
Increases the no. of B adrenergic receptors
Effects of thyroid hormones on the gastrointestinal system
Thyroid disease is common and it affects the gastrointestinal (GI) system. In hypothyroidism, the most common GI symptoms are constipation and anorexia. Meanwhile, diarrhea and increased appetite are often observed in hyperthyroidism
Effects of thyroid hormones on the Growth and Development
•Promote expression of the gene for growth hormone in somatotrophs.
•T3 is required for the production of both growth hormone and insulin-like growth factors. It is also required for the action of insulin-like growth factors on epiphyseal cartilage.
•They stimulate the linear growth of bones and endochondral ossification and maturation of epiphyseal bone centers.
Effects of thyroid hormones on the reproductive system
Required for the normal ovarian cycle of follicular development, maturation, and ovulation, maintenance of the healthy pregnant state, and the homologous testicular process of spermatogenesis.
• Deleterious effects may be caused by alterations in the metabolism or availability of steroid hormones – thyroid hormones stimulate hepatic synthesis and release of sex steroid-binding globulin.
In female
Lack of thyroid function
– Menorrhagia
– Polymenorrhea, Decreased libido
In hyperthyroid woman
– Oligomenorrhea
– Amenorrhea (occasionally)
In male
– Lack of thyroid hormone-loss of libido
-Great excess—impotence
Effect of thyroid hormones on BMR
Extreme excess of thyroid secretion increases BMR to 60-100% above normal
Complete lack of thyroid secretion causes BMR to fall 40-50% below normal
Mechanism of increasing metabolic rate and temperature
By increasing the number, size, and activity of mitochondria
By increasing plasma membrane Na+-K+ ATPase Activity
By stimulating both catabolic and anabolic reactions in pathways affecting fats, carbohydrates, and proteins.
Effects secondary to increased metabolism
Thyroid hormone increases O2 consumption in almost all the cells – exceptions are the adult brain, testes, uterus, lymph nodes, spleen, and anterior pituitary.
Nitrogen excretion is increased
Weight loss due to catabolism of endogenous protein and fat stores
Increased urinary excretion of K+, uric acid
Precipitation of vitamin deficiency syndrome due to the increased need for vitamins resulting from an increased metabolic rate
