2. Thyroid Gland & Disorders

What is the structure of the thyroid gland & its hormone-related cells?

• 2 lobes beside trachea, linked by isthmus

• Made of colloid-filled follicles with follicular cells

• Follicular cells use thyroglobulin to make thyroid hormones

• Parafollicular (C) cells secrete calcitonin to regulate calcium

How is thyroid hormone release regulated?

• Hypothalamus releases TRH → stimulates anterior pituitary

• Anterior pituitary releases TSH → stimulates thyroid follicular cells

• Thyroid releases T4 → converted to active T3

• ↑T3 triggers negative feedback on pituitary → ↓TSH

• No direct feedback from T3/T4 to hypothalamus, only to pituitary

How are thyroid hormones T3 & T4 synthesised & how do they act?

• Thyroglobulin (prohormone) secreted by follicular cells into follicle

• Iodine enters via Na⁺/I⁻ pump & binds thyroglobulin → forms T3 & T4

• T3 & T4 enter blood → T4 converted to active T3

• T3 enters nucleus, binds receptor → triggers protein synthesis & cell response

What is the role of thyroid hormones in metabolism?

• ↑ Basal Metabolic Rate (BMR) in all tissues

• Carb metabolism: ↑ sensitivity to catecholamines, insulin & GH; ↑ gluconeogenesis, glycogenolysis & glucose use

• Protein metabolism: low T3/T4 = protein synthesis; high T3/T4 = protein breakdown & weight loss

• Fat metabolism: ↑ lipolysis, ↑ liver uptake of cholesterol = ↓ plasma cholesterol

What is the role of thyroid hormones in thermoregulation (calorigenesis)?

• ↑ O₂ consumption in most tissues (except brain, spleen, testes)

• ↑ heat production via ↑ Na⁺/K⁺ ATPase activity

What is the role of thyroid hormones in CNS maturation?

• Essential for brain development during late foetal & early postnatal life

What is the role of thyroid hormones in skeletal growth & maturation?

• Essential for normal bone growth, maturation & development of adult stature

• Supports development of CVS, GI tract, teeth, skin & hair follicles

What are the causes of hypothyroidism?

• Surgical removal (e.g. partial thyroidectomy)

• Radioactive iodine (¹³¹I) treatment

• Pituitary or hypothalamic disease → ↓ TSH

• Iodine deficiency due to low dietary intake

• Autoimmune thyroiditis (e.g. Hashimoto’s)

What are the features, diagnosis & treatment of congenital hypothyroidism (cretinism if untreated)?

• Features: dwarfism, protruding tongue & abdomen, mental retardation, coarse scanty hair, dry yellowish skin

• Diagnosis: TSH assay from heel-prick blood sample

• Treatment: thyroxine replacement therapy

What are the features and treatment of juvenile hypothyroidism?

• Features: stunted growth, delayed sexual development, mental retardation

• Treatment: thyroxine replacement therapy

What are the causes, symptoms, and treatment of adult hypothyroidism (Hashimoto's thyroiditis)?

• Causes: autoimmune destruction of the thyroid gland

• Symptoms:

  • Painful swelling (goitre)

  • Low BMR (slow bodily processes, slow movements)

  • Weight gain, cold intolerance

  • Decreased sweating, constipation

  • Myxoedema (hair loss)

  • Fatigue, memory impairment, depression, slow speech

  • Bradycardia

  • Yellowish skin (due to increased plasma carotene)

  • Low serum T3/T4, high TSH, and possible elevated plasma cholesterol

• Treatment: lifelong thyroxine replacement (Levothyroxine sodium, Liothyronine sodium)

What are the causes of hyperthyroidism?

• Excess thyroid hormone intake (thyrotoxicosis factitia)

• Follicular thyroid carcinoma

• Amiodarone treatment

• Primary hyperthyroidism (Graves’ disease – autoimmune)

What are the causes, symptoms, and hormonal findings in hyperthyroidism (Graves’ disease)?

• Causes: thyroid-stimulating immunoglobulins bind to TSH receptors

• Symptoms:

  • High BMR → heat intolerance, warm skin, excessive sweating

  • Weight loss

  • Palpitations, tachycardias

  • Restlessness, anxiety, irritability

  • Exophthalmos (eye changes)

  • Diffuse toxic goitre

• Hormonal findings: high serum T3/T4, low serum TSH

What are the treatment options for Graves’ disease?

• Antithyroid drugs

• Radioactive iodine

• Surgery

What are the treatment options for Graves’ disease & their key features?

Antithyroid drugs:

• Carbimazole, Propylthiouracil (PTU)

• Used for long-term treatment, pre-radioiodine & pre-surgery

• Side effects: agranulocytosis (↓ WBC, flu-like symptoms), hepatotoxicity

• Lugol’s iodine (aqueous iodine solution BP):

  • 5% iodine in 10% potassium iodide

  • Used pre-surgery to ↓ thyroid size & vascularisation

• β-adrenoceptor blockers:

  • Propranolol, nadolol, sotalol

  • Symptomatic relief (↓ palpitations & arrhythmias)

How does radioactive iodine (¹³¹I) treat Graves’ disease & what are the precautions?

• Given orally as Na¹³¹I solution

• Selectively destroys overactive thyroid tissue via local irradiation

• Acts over 1–6 months

• Patient must be euthyroid before treatment (i.e. must first be stabilised on antithyroid drugs to normalise T₃/T₄ levels)

• Contraindicated in pregnancy & very young children

When is partial thyroidectomy used to treat Graves’ disease & what are the requirements?

• Used when antithyroid drugs or radioiodine aren't suitable

• It's a demanding surgical procedure

• Patient must be euthyroid before surgery (i.e. stabilised on antithyroid drugs to normalise T₃/T₄ levels)