Veterinary Physiology-III

HYPOTHYROIDISM

• Goitre means enlarged thyroid gland.
• Insufficient I₂ intake in animals and man leads to goitre known as endemic goitre.
• Lack of I₂ reduces thyroid hormone synthesis and the reduced blood TH enhances TSH from adenohypophysis. The TSH stimulates the thyroid gland to secrete excess thyroglobulins into the follicles and the gland grows very large. But the thyroid hormone secretion cannot be increased due to lack of I₂ leading to further increase in TSH secretion. Hence this condition is called as endemic colloid goitre.
• Mild thyroiditis causes slight hypothyroidism which leads to increased TSH secretion and the gland enlarges producing non-toxic or colloid goitre.
• Hypothyroidism is common in dogs.
• Ingestion of goitrogenic substances also causes goitre.

Symptoms

• Fatigue, somnolence, muscular sluggishness, reduced heart rate, decreased blood volume, increased weight, constipation, mental sluggishness, hair falling and edematous appearance of whole body, bagginess of eyes, and swelling of face. Increased quantities of mucoplysaccharides accumulate in the interstitial spaces leading to myxedema.
• When hypothyroidism is severe during foetal life, infancy and in young age especially in humans, there is failure of body growth and CNS development is impaired with brain hypoplasia and this condition is called as cretinism – physical and mental growth are retarded; skeletal growth is inhibited, soft tissues enlarge greatly and the cretin appears obese and stocky.

Treatment

• Oral ingestion of thyroid hormone.
• Hypothyroidism results in retarded growth while in hyperthy­roidism, excessive skeletal growth in young animals.