Thyroid hormones affect basal metabolic rate, regulate long bone growth, affect protein synthesis, and promote neuronal maturation.
The thyroid gland also develops the nervous system, increases metabolism of carbohydrates, affects heart beat by increasing the number of beta-adrenergic receptors, whilst also affecting the force of the heart’s contractions by affecting the proportions of cardiac proteins, thyroid also stimulates consumption of oxygen by metabolically active tissues including the brain, pituitary, spleen, lymph nodes, and increases metabolic rate.
The thyroid gland weighs approximately 20g and is therefore one of the largest endocrine glands. It is larger in females than males.
The gland itself is a butterfly shaped gland with the two lobes joined in the lower centre by the ‘isthmus’ a band of thyroid tissues. It is situated beneath the laynx and extends from vertebra C5 to T1. it is contained within the perithyroid sheath, the ligament of Berry attaches it to the cricoid cartilage.
The thyroid gland contains spherical follicles which contain walls consisting of a layer of cuboidal epithelial cells. The lumen of each follical contains thyroglobulin, a protein – thyroid hormones bind to this.
Blood supply is by the bilateral carotid arteries, the supercriothyroid artery and subclavian system. Venous drainage occurs through the superior, middle and inferior thyroid veins.
The gland has both sympathetic and parasympathetic autonomic nerves. The exterior laryngeal nerve is a branch of the vagus nerve which supplies the cricothyroid muscle. Damage to this nerve affects speech.
The parathyroid glands are situated on either side of the thyroid gland. Parathyroid hormone and vitamin D regulates concentrations of calcium and phosphorus. The thyroid gland produces triodothyronine (T3) and thyroxine (T4) which have a role during development and metabolic homeostasis in adults.
T4 is only produced in the thyroid gland. 80% of T3, however, is prodced when T4 is concerted to T3 in the peripheral tissues. Only 20% of T3 comes from the thyroid gland itself. Synthesis of thyroid hormone depends on adequate intake of dietary iodine.
Thyroid hormone sysnthesis is also dependent on TSH which is produced in the anterior lobe of the pituitary gland. Thyroid hormones attach to thyroglobulin in the thyroid follicles.
TRH (thyrotropin releasing hormone) is generated in the hypothalamus and with TSH increases the secretion of T3 and T4 in the thyroid gland. When TRH increases, so does the secretion of TSH. As T3 and T4 increase, the secretion of TSH an TRH decreases.
TSH is an accuate test as levels of TSH are dramatically affected by changes in T3 and T4. if TSH is found to be abnormal, circulating thyroid hormone levels must be checked to consider hyper or hypo thryoidism.
Thyrotoxicosis is caused by abnormally high levels of circulating T3 and T4, and is generally caused by hyperthyroidism. This is often treated by radioactive iodine to slow the hormonal production, or anti-thyroid drugs to prevent synthesis of thyroid hormones. If this is ineffective partial or total removal of the thyroid gland may then be reconsidered.
Graves disease is charactised by thyrotoxicosins, infiltrative opthalmopathy and infiltrative dermopathy. It is more common in women than men and most prevalent in people between 20-40 years. Generally the thyroid gland is enlarged in such cases. Graves disease is indicated with raised serum T3 and T4 and decreased TSH.
Hyperthyroidism often manifests as tachycardia, weight loss, thyroid hypertrophy, tremors, muscle weakness, lower extremity oedema.
Goitres are generally caused by dietary iodine deficiency (found in eggs, fish, seaweed and cheddar cheese). It leads to a rise in TSH.