Thyroid hormones (T3, T4) are synthesised in the thyroid gland from the main substrate, which is iodide (I¯).

Histology of the thyroid gland

• The thyroid gland consists of follicles, which are the units that produce thyroid hormones
• Follicles contain follicular cells arranged in a circular fashion around the colloid.
• The capillaries delivering the iodine and nutrients face the basolateral membrane of the follicular cells, while the apical membrane faces the colloid.
• C cells are interspersed between follicles, which produce calcitonin

Transport of thyroid hormones

• Greater than 99% of T3 (99.5%) and T4 (99.95%) are bound to serum proteins when they enter the circulation and only <0.5% remain free
• Thyroxine-binding globulin (TBG) transports ~75% of thyroid hormone, transthyretin ~15% and albumin ~10%. Only the hormones free in plasma are biologically active, while those that are protein-bound act as a reserve.

Thyroid Cancer

Hypothyroidism

Hyperthyroidism

Venn diagram showing how different causes of thyroid dysfunction may manifest. Note how many causes of hypothyroidism may have an initial thyrotoxic phase.

Thyroid in Pregnancy

Thyroid: Thyroiditis

Thyroid - Neoplasia

Thyroid - Hyperthyroid

Thyroid