Abstract
Highly vascular endocrine gland, situated at the level of fifth cervical to first thoracic vertebrae. It usually weights 25 g, but is slightly heavier in Women and enlarges during menstruation and pregnancy.
1 Thyroid Gland
Highly vascular endocrine gland, situated at the level of fifth cervical to first thoracic vertebrae. It usually weighs 25 g, but is slightly heavier in Women and enlarges during menstruation and pregnancy.
The thyroid gland consists of two lobes and interconnecting isthmus.
Blood supply: superior thyroid and inferior thyroid artery. In 10% of the population there is a thyroid ima artery, which has a variable origin. Superior, inferior and middle thyroid veins form a thyroid plexus of veins on the anterior surface of the gland. Fourth thyroid vein of Kocher may be present sometimes. See Figure 5.1.
Microstructure:
◦ The thyroid gland consists of a number of follicles, which are the functional units of thyroid.
◦ The follicles consist of a central colloid core surrounded by epithelium, which varies from squamous to cuboidal to columnar, depending on their level of activity.
◦ Follicular cells secrete thyroid hormones triiodothyronine (T3) and tetraiodothyronine (T4) . These hormones regulate tissue basal metabolism and heat production, and also regulate the development of the nervous system in the fetus.
◦ C cells in the thymus secrete calcitonin, which helps to lower the blood calcium.
Applied:
◦ Goitre is enlargement of the thyroid gland, caused either due to hypothyroidism or hyperthyroidism.
◦ Hypothyroidism can be due to insufficient dietary iodine causing iodine deficiency/endemic goitre, or it can be an autoimmune disease such as Hashimoto’s thyroiditis.
◦ Adolescent goitre/puberty goitre: enlargement of thyroid gland during puberty induced by increase in hormone requirement. It often resolves spontaneously as the period of maximal hormonal activity passes.
◦ Hyperplasia of the thyroid gland occurs during pregnancy, due to which there may be slight enlargement of the gland but the hormone secretion is normal.
◦ Hypothyroidism leads to menorrhagia and polymenorrhoea. However, in some cases, irregular periods or even amenorrhoea can occur. This occurs because low T3 and T4 stimulates the hypothalamus to produce thyrotropin-releasing hormone, which in turn stimulates thyroid stimulating hormone (TSH) and prolactin production in the pituitary. Hence, increased prolactin inhibits gonadotropin-releasing hormone release needed for luteinising hormone (LH) and follicle stimulating hormone (FSH) production, causing abnormal ovarian function.
◦ Congenital hypothyroidism (also referred to as cretinism): caused by extreme hypothyroidism during childhood. In normal pregnancy, thyroid hormones cross the placental barrier and are critical in the early stages of fetal brain development. The fetal thyroid starts functioning at 14 weeks of gestation. If maternal hypothyroidism is present before the development of the fetal thyroid gland, the intellectual disability is severe. Thyroid hormones also stimulate gene expression of growth hormone in somatotrophs. Therefore, generalised stunted body growth is seen in hypothyroidism.
