Functional unit of the thyroid

Follicle

• Epithelial cells surround central colloid
• Follicular cells lie adjacent to a vein / capillary
• Produces 80% T4 and 20% T3 (rest of T3 is converted in the peripheries)
• T3 is the active form

Parafollicular cells

• Secrete calcitonin

T3/T4 Physiology

1. Iodine transported across cell membrane, transfered to colloid, then linked to tyrosine
2. Tyrosine iodinated to mono-iodotyrosine + di-iodotyrosine (bound to thyroglobulin)
3. Coupling results in T3 and T4
4. T3/T4 + thyroglobulin transported into cell as colloid droplets; then separated from TG
5. T3/T4 released into circulation (bound to albumin, thyroxine binding globulin [different from thyroglobulin])

Goitre

• Any enlargement of the thyroid gland
• Seen as midline swelling that moves with swallowing

Hyperthyroidism

1. Immune
   • Graves
   • Hashimoto's
2. "Physiological"
   • Pregnancy
3. Drugs
   • Thyroxine replacement (in block and replace)
   • Amiodarone
4. Carcinoma
   • hyperactive nodule
   • Follicular carcinoma
   • Papillary carcinoma
   • Struma ovarii - thyroid tissue in dermoid cyst
   • Basophil adenoma of pituitary

Thyroid disease workup

2. Radioactive scanning (not a very useful test really is it?!)
   • Hot nodule: 95% benign
   • Cold nodule: 80% benign
3.