Thyroid Gland

Thyroid Physiology

TSH (from pituitary) stimulates thyroid to produce:

T4 (thyroxine) – mainly secreted form

T3 (triiodothyronine) – active form (mostly converted peripherally)

Bound to thyroxine-binding globulin (TBG) in plasma

Negative feedback: T3/T4 suppress TSH and TRH

Thyroid Function Tests (TFTs)

Primary hypothyroidism: ↑ TSH, ↓ T4

Primary hyperthyroidism: ↓ TSH, ↑ T4/T3

Subclinical: TSH abnormal, T4/T3 normal

TBG changes: ↑ in pregnancy/oestrogen (↑ total T4), ↓ in nephrotic syndrome/liver disease

Hyperthyroidism

Causes

Graves’ disease (autoimmune TSH receptor antibodies)

Toxic multinodular goitre

Toxic adenoma

Thyroiditis (initial hyperthyroid phase)

Exogenous thyroxine intake

Features

Weight loss, heat intolerance, palpitations, tremor, anxiety

Eye signs in Graves’: proptosis, lid lag, ophthalmoplegia

Goitre ± bruit

Atrial fibrillation, osteoporosis, hyperreflexia

Management

Carbimazole or propylthiouracil (PTU):

PTU preferred in pregnancy (1st trimester)

Risk: agranulocytosis (monitor for sore throat, fever)

Radioiodine therapy (CI in pregnancy, eye disease)

Surgery if large goitre or treatment failure

β-blockers for symptomatic relief

Hypothyroidism

Causes

•    Primary: Hashimoto’s thyroiditis (anti-TPO antibodies), iodine deficiency, post-thyroidectomy/radioiodine

•    Secondary: pituitary/hypothalamic disease (↓ TSH/TRH)

•    Drugs: lithium, amiodarone, carbimazole/PTU

Features

•    Weight gain, cold intolerance, lethargy, bradycardia

•    Constipation, dry skin, menorrhagia, hoarse voice

•    Non-pitting oedema (myxoedema), delayed reflexes

Management

•    Levothyroxine (start low in elderly/ischemic heart disease)

•    Monitor with TSH (target: normal range)

Thyroiditis

•    Subacute (de Quervain’s): painful thyroid, post-viral, ↑ ESR, transient hyperthyroid → euthyroid → hypothyroid

•    Silent/painless (lymphocytic): autoimmune, seen postpartum

•    Hashimoto’s: firm goitre, anti-TPO positive, hypothyroid

Thyroid Nodules & Cancer

•    Thyroid nodule work-up: TFTs → USS → FNA (if suspicious)

•    Benign features: cystic, hyperechoic, comet tail artefact

•    Malignancy types:

o    Papillary (most common): spreads via lymph, good prognosis

o    Follicular: spreads haematogenously

o    Medullary: calcitonin-producing, MEN2 association

o    Anaplastic: aggressive, elderly, poor prognosis

•    Management: surgery ± radioiodine ± thyroxine suppression

Miscellaneous

•    Sick euthyroid syndrome: ↓ T3, normal/↓ T4, normal/↓ TSH (no treatment)

•    Thyroid storm: life-threatening thyrotoxicosis → fever, tachycardia, delirium; treat with β-blockers, steroids, PTU, iodine