Lecture 5-7 Thyroid , Parathyroid
Dec 28, 2015
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
http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/thyroid/anatomy.html
THYROID GLAND HISTOLOGY
Histology of Parathyroid Gland
• Principal (Chief) - cells produce parathyroid hormone (PTH); more numerous
• Oxyphil cell - function is unknown
Actions of thyroid hormones:
T3 and T4 have the following effects:
1- General metabolism: Thyroid hormones increase the metabolic rate and O2 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 B12 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
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
• Hormones
– Parathyroid hormone
(PTH)
– Calcitonin (CT)
– Vitamin D3
• Target organs
– Bone
– Intestinal tract
– Kidney
Endocrine regulation of Calcium & phosphate
metabolism
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.
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
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.
www.hsc.missouri.edu/~daveg/thyroid/thy_dis.html
EXAMPLES OF THYROID DISEASES
1° Hypothyroidism Hyperthyroidism
Disorders of thyroid glandA- Hypothyroidism
Hypothyroidism is a syndrome resulted from a decrease in the secretion of thyroid hormones T3 & T4 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
www.hsc.missouri.edu/~daveg/thyroid/thy_dis.html
Congenital HypothyroidismJuvenile Hypothyroidism