Glossary of Physiology Terms

Goiter

Goiter is an enlargement of the thyroid gland caused most commonly by iodine deficiency in the diet. Iodine deficiency leads to low levels of thyroid hormone production, and a reduction in thyroid hormone negative feedback on the hypothalamus and the anterior pituitary leads to a compensatory rise in the thyroid stimulating hormone (TSH). Enlargement of the thyroid gland is, therefore, caused by abnormally high circulating levels of TSH, which has a strong trophic effect on the thyroid gland.

Primary hypothyroidism

Primary hypothyroidism is characterized by abnormally low levels of thyroid hormone (T₃ and T₄) production, where the defect is at the level of the thyroid gland itself.

In primary hypothyroidism, the circulating levels of the thyroid hormones are low, however, the levels of the thyroid stimulating hormone (TSH) are high due a lack of thyroid hormone negative feedback on the anterior pituitary.

Common causes of primary hypothyroidism include iodine deficiency (which leads to goiter), and Hashimoto's disease, in which autoimmune antibodies destroy the ability of the thyroid gland to produce the thyroid hormones.

Hypothyroidism
Secondary hypothyroidism
Tertiary hypothyroidism