Lecture 5-7 Thyroid, Parathyroid

Thyroid Embryology  Derived from endodermal tissue at base of tongue  Embryonal remnants form Thyroglossal duct; pyramidal lobe; lingual thyroid  Fuse with C-cells (neural crest origin), derived from the the 5th branchial arch  C-cells scattered through posterior/superior lobes

Thyroid Gland Parathyroid Glands

Thyroid Gland Trachea Esophagus Thyroid

Thyroid Gland calcitonin = thyrocalcitonin

THYROID GLAND HISTOLOGY

Thyroid Gland

Histology of Parathyroid Gland Principal (Chief) - cells produce parathyroid hormone (PTH); more numerous Oxyphil cell - function is unknown

Thyroid & Parathyroid, 400x

Actions of thyroid hormones: T 3 and T 4 have the following effects: 1- General metabolism: Thyroid hormones increase the metabolic rate and O 2 consumption of all tissues of the body except the adult brain, lungs, lymphoid tissues, retina and anterior pituitary gland. 2- Protein metabolism: Normal levels of thyroid hormones stimulate protein synthesis (anabolic effect) whilst excess Thyroid hormones secretions cause protein catabolism (breakdown of protein molecules). 3- Carbohydrate metabolism: Thyroid hormones stimulate glucose uptake and utilization by tissues, insulin hormone secretion, liver glycogenolysis and intestinal absorption of glucose. 4- Lipid metabolism: Thyroid hormones decrease the level of lipid and cholesterol in the blood (i.e. increased cholesterol catabolism than lipogenesis)

Actions of thyroid hormones (Continued) 5- Vitamin metabolism: Thyroid hormones are essential for hepatic conversion of carotene (in green vegetable and carrot) into vitamin A. 6- Growth and maturation: Thyroid hormones are essential for physical, mental and sexual growth. 7- Cardiovascular manifestations: Tachycardia (i.e. increased heart rate). The strength of myocardial muscle is increased. Increased cardiac output. Increased systolic blood pressure with a concomitant decrease in diastolic blood pressure. So, pulse pressure is increased. 8- Respiratory system: Thyroid hormones cause an increase in rate and depth of breathing to increase body metabolism. 9- Gastrointestinal tract: Thyroid hormones increase appetite, digestive juice secretion, intestinal motility and absorption. 10- Hemopoietic system: Thyroid hormones stimulate erythropioesis by increasing the rate of metabolism in the bone marrow and enhance absorption of vitamin B 12 from terminal ileum.

Thyroid Hormones  The thyroid produces T3 and T4  T4 ( Tetraiodothyronine )  T3 ( Triiodothyronine ), Reverse T3 T3T4

Calcitonin Peptide hormone, acts opposing to PTH Secreted by parafollicular or C cells of thyroid gland Function : Decrease plasma concentration of calcium Acts on bone & kidney Regulation of secretion – plasma Ca concentration

Thyroid hormone synthesis

Functions of thyroid hormones Generally cause increased BMR It increases transcription of large number of genes Thyroid hormone receptors are either attached to DNA or located near to it

SPECIFIC ACTIONS OF THYROID HORMONE: METABOLIC Regulates of Basal Metabolic Rate (BMR). Increases oxygen consumption in most target tissues. Permissive actions: TH increases sensitivity of target tissues to catecholamines, thereby elevating lipolysis, glycogenolysis, and gluconeogenesis.

Thyroid (cont) Regulates basal metabolic rate Improves cardiac contractility Increases the gain of catecholamines Increases bowel motility Increases speed of muscle contraction Decreases cholesterol (LDL) Required for proper fetal neural growth

Regulation of PTH Secretion

Calcium Regulation Calcium homeostasis Total blood calcium is usually 10 mg/dl

PTH and Calcium Feedback Loop

BASICS OF THYROID HORMONE ACTION IN THE CELL

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Calcium Homeostasis

Hormones –Parathyroid hormone (PTH) –Calcitonin (CT) –Vitamin D3 Target organs –Bone –Intestinal tract –Kidney Endocrine regulation of Calcium & phosphate metabolism

Thyroid abnormalities 15.3 Thyroid and parathyroid glands

Thyroid & parathyroid pathophysiology –Hypothyroidism During childhood = cretinism (“cretins”) During adulthood = myxedema –Goiter (abnormal thyroid growth) Endemic = inadequate iodine intake Grave’s disease

Goiter

Goiter is an enlargement of thyroid gland. It may be accompanied with normal, hypo function or hyper function of the thyroid. Types of goiter 1.Simple (non – toxic) goiter: It is non – inflammatory, non – neoplastic (not tumor) enlargement of the thyroid gland, that is not initially associated with hypo or hyper secretion of thyroid hormones (hyperthyroidism). The thyroid gland enlarged with normal thyroid function (i.e. euthyroidism). It is due to insufficient iodine in water and soil for food stuffs. It may occur physiologically with the increase demand of the body to thyroid hormones e.g. at puberty and during pregnancy.

-cretinism: A type of mental retardation and bodily malformation caused by severe, uncorrected thyroid deficiency in infancy and early childhood.  Hyperthroidism -Grave’s disease: Grave’s disease is an autoimmune disease in which the immune system produces antibodies which stimulate the TSH receptors of the thyroid gland, resulting in overproduction of thyroid hormones. Symptoms: increased pulse rate, increased sweating, heat intolerance, hair loss, inflammation of the eyes, swelling of the tissues around the eyes, and protrusion of the eyes.

Myxedema

Physical Findings for Myxedema Comatose or semi comatose Dry coarse skin Hoarse voice Thin dry hair Delayed reflex relaxation time Hypothermia Pericardial, pleural effusions, ascites

Periorbital Myxoedema

Myxedema causing puffiness of the face in an adult with hypothyroidism. Disorders of thyroid function  Hypothyroidism -Myxedema - Hypothyroidism occurring in adulthood. Clinical features include lethargy, cold intolerance, decreased sweating, bradycardia, tongue enlargement, and non-pitting edema of the skin due to infiltration of the subcutaneous tissues by metachromatic proteoglycans.

EXAMPLES OF THYROID DISEASES 1° Hypothyroidism Hyperthyroidism

Disorders of thyroid gland A- Hypothyroidism Hypothyroidism is a syndrome resulted from a decrease in the secretion of thyroid hormones T 3 & T 4 or inability of the tissues to use the secreted hormones. Hypofunction of the thyroid gland during childhood is called '‘Cretinism'' and if occurs during adulthood, it is called '' Myxoedema''. Cretinism Cretinism is a hypothyroidism in children. The child who is hypothyroid since birth is called ''Cretin'' Causes of cretinism: a) Congenital absence of the thyroid b) Iodine deficiency during pregnancy.

EXAMPLES OF THYROID DISEASES Congenital HypothyroidismJuvenile Hypothyroidism