1 - Thyroid disorders

Abstract

This chapter discusses thyroid disorders. The thyroid gland secretes its principal hormones, thyroxine (T 4 ) and tri-iodothyionine (T 3 ) into the circulation where they are associated with two proteins that bind them specifically. T 3 is the metabolically active form of thyroid hormone. Both T 4 and T 3 are largely bound to plasma proteins. It is the free hormone that is metabolically active. Changes in the concentration of the serum binding protein will lead to parallel changes in the serum values of total thyroid hormones, but the free thyroid hormone values remain constant. Hypothyroidism is the clinical condition resulting from decreased circulating concentrations of free (unbound) thyroid hormones. Hypothyroidism is commonly caused by primary thyroid failure when serum thyrotrophin concentration is high because of a lack of negative feedback at the pituitary level by thyroid hormones. It can also be secondary to pituitary failure of thyrotrophin secretion when serum thyrotrophin values are usually low or just within the normal range. Hyperthyroidism is the clinical condition resulting from increased circulating serum concentrations of free thyroid hormones. Symptoms include weight loss, heat intolerance, tachycardia, and lid retraction. The main causes of hyperthyroidism are Graves' disease, toxic multinodular goitre, toxic nodule, painless thyroiditis, and trophoblastic thyroid stimulating hormone syndrome, but the condition can be induced by over-treatment with thyroid hormones.