HORMONES AND HORMONE ANTAGONISTS: A TEACHER’S COMPREHENSIVE GUIDE
Welcome, future pharmacologists and healthcare professionals!
The word ‘Hormone’ has originated from a Greek word ‘Hormaein’ which means ‘to impel’. Thus, a hormone is a substance which is secreted by specialised cells and transported to a distant site to exert its action upon specific tissues. Hormones are synthesised and discharged by endocrine glands directly into the blood-circulation without the intervention of a duct—therefore known as ductless glands.
Hormone antagonists are chemical substances that prevent the function of the endocrine glands, the biosynthesis of their secreted hormones, or the action of hormones upon their specific sites. As a pharmacology educator with years of experience teaching endocrine pharmacology, I have observed that students often find this topic challenging due to the complex feedback mechanisms and numerous hormones. Let me tell you: Understanding hormones and their antagonists is essential for managing endocrine disorders.
In this comprehensive guide, I will walk you through the major hormones and hormone antagonists—thyroid hormones, anti-thyroid drugs, parathormone, calcitonin, vitamin D, insulin, oral hypoglycaemic agents, estrogens, progesterone, oxytocin, and corticosteroids. By the end of this article, you will have a thorough understanding of these essential pharmacological agents. Let us begin!
Dpharmguru’s exam insights:
Endocrine pharmacology is frequently tested in pharmacy exams. Remember: Thyroid hormones (T₃, T₄) regulate metabolism. Anti-thyroid drugs (PTU, Carbimazole) treat hyperthyroidism. Parathormone increases calcium; Calcitonin decreases calcium. Insulin lowers blood glucose; Oral hypoglycaemics include Sulfonylureas, Biguanides, and α-Glucosidase inhibitors. Estrogens and Progesterone are female sex hormones. Corticosteroids include Glucocorticoids and Mineralocorticoids. These are classic exam questions!
HORMONE ANTAGONISTS
- LH-RH Antagonist: Buserelin, Leuprolide—decrease oestrogen and testosterone formation; used in prostatic and breast cancers
- Antiandrogen: Flutamide—antagonises androgen action; used in prostate carcinoma
- Antioestrogen: Tamoxifen, Nafoxidine, Clomiphene—used in advanced breast cancer (postmenopausal women)
- 5-α Reductase Inhibitor: Finasteride—prevents conversion of testosterone to dihydrotestosterone; used in advanced prostate carcinoma
THYROID HORMONES
TSH (Thyroid Stimulating Hormone) released by the anterior pituitary gland regulates the secretion of thyroxine (T₄), triiodothyronine (T₃), and thyrocalcitonin (TCT) from the thyroid gland.
Thyroxine (T₄)
Clinical Uses: Regulates BMR, controls urine output, helps in homeothermy, stimulates protein synthesis, regulates mental development, increases action of adrenaline and noradrenaline.
Triiodothyronine (T₃)
Clinical Uses: Stimulates RNA polymerase I and II—increases protein synthesis; increases glycogen breakdown and glucose synthesis; stimulates cholesterol breakdown; increases heart rate and force of contraction.
Physiological Role of Thyroid Hormones
- Lipid Metabolism: Increase fatty acid oxidation; plasma cholesterol and triglycerides inversely depend on thyroid hormone levels
- Carbohydrate Metabolism: Enhance insulin-dependent glucose entry; increase gluconeogenesis and glycogenolysis
- Growth: Essential for normal growth of children
- Development: Necessary for foetal and neonatal brain development
- CVS: Increase cardiac output, contractility, and heart rate; promote vasodilation
- CNS: Mental sluggishness in hypothyroidism; anxiety in hyperthyroidism
- Reproductive System: Normal function depends on thyroid hormone levels
- GIT: Diarrhoea in hyperthyroidism; constipation in hypothyroidism
Pathological Role and Clinical Uses of Thyroid Hormones
- Hypothyroidism (Myxoedema): Lifelong thyroid hormone replacement
- Hypothyroid Coma: Emergency—L-thyroxine (500 µg) IV; Liothyronine (5-10 µg) via orogastric tube
- Cretinism: Prevented by early thyroid therapy (L-thyroxine 25 µg initial dose)
ANTI-THYROID DRUGS
Classification
- Hormone Synthesis Inhibitors: Carbimazole, Methimazole, Propylthiouracil
- Hormone Release Inhibitors: Iodine, Iodides of Na and K, Organic Iodide
- Ionic Inhibitors: Perchlorates, Thiocyanates, Nitrates
- Radioactive Iodine: Iodine (123, 125, 131)
Propylthiouracil (PTU)
Indications: Hyperthyroidism (Grave’s disease), pre-surgical thyroid removal, before radioactive iodine therapy.
Adverse Effects: Agranulocytosis (decreased WBC count), thrombocytopenia.
Iodine and Iodides
Indications: Thyroid surgery, thyroid crisis, endemic goitre, expectorant, topical antiseptic.
Adverse Effects: Acute reaction (angioedema, fever, joint pain); Chronic overdose—Iodism (inflammation of mucous membranes, salivation, rhinorrhoea, sneezing, lacrimation).
Radioactive Iodine (RAI)
Indications: Radioactive iodine uptake test (evaluation of thyroid function), treatment of thyrotoxicosis and toxic adenomas.
Adverse Effects: Delayed hypothyroidism.
Carbimazole