Hypothyroidism and Hyperthyroidism

Hypothyroidism

Causes

1. Insufficient thyroid tissue
   • Developmental disorders
   • Radiation and surgical injury
   • Thyroiditis (e.g., Hashimoto's) with destruction of tissue

2. Inhibition of hormone synthesis
   • Idiopathic primary hypothyroidism (autoimmune TSH receptor blockade?)
   • Inborn errors of metabolism
   • Iodine deficiency
   • Drugs (lithium, iodides and others)
   • Thyroiditis (e.g., Hashimoto's)

3. Central
   • Loss of pituitary tissue with decreased TSH secretion
   • Hypothalamic lesions with decreased TRH secretion

Clinical sequelae

1. Myxedema
   • In adults
   • Lethargy, cold intolerance, apathy, slow speech and mentation
   • Periorbital edema, dry coarse skin, thickened tongue, constipation
   • Flabby enlarged heart with pericardial effusion (hypothyroid cardiomyopathy)
   • In elderly patients, may be confused with "senility"

2. Cretinism
   • In children
   • Retarded mental and physical growth
   • Classic appearance: dry, rough skin, periorbital puffiness, wide-set eyes, broad nose, protuberant tongue
   • May occur with developmental failure of the thyroid, inborn errors of metabolism affecting thyroid hormone synthesis, or iodine deficiency during fetal development

Hyperthyroidism

Causes

1. Grave's Disease (autoimmune stimulation of the TSH receptor)
2. Toxic multinodular goiter (toxic refers to excess thyroid hormone production)
3. Toxic thyroid adenoma
4. Other thyroid, ovarian, or placental tumors are uncommon causes
5. Iatrogenic (exogenous thyroxine)

Clinical presentation: Thyrotoxicosis

• Hypermetabolic state
• Tachycardia, palpitations, tremor, diarrhea
• Heat intolerance, warm skin, perspiration, nervousness
• Thyrotoxicosis + hyperfunction of the thyroid gland = hyperthyroidism

  Thyrotoxicosis can also result from intermittent release of excess thyroid hormone from a thyroid gland that is not hyperfunctional. For example, destruction of thyroid tissue in thyroiditis (e.g., Hashimoto's) occasionally causes release of excess thyroid hormone from colloid as follicles break down. This produces transient symptoms of thyrotoxicosis, but is not classified as hyperthyroidism.