8.7: Disorders of Thyroid Function- Hypo- and Hypersecretion

Low iodine or autoimmune diseases can disrupt normal thyroid hormone production, causing either hypothyroidism hyperthyroidism.

Master this section and you'll be able to

• Describe how inadequate dietary iodine impairs T₃/T₄ synthesis and leads to elevated TSH and goiter.
• Compare iodine deficiency hypothyroidism to autoimmune thyroiditis (both: Hashimoto's thyroiditis and Graves disease).
• Differentiate hypothyroidism from hyperthyroidism by relating hormone levels to key clinical signs.

Iodine Deficiency

As you have learned, dietary iodine is essential for the production of the thyroid hormones T₃ (triiodothyronine) and T₄ (thyroxine), which regulate metabolism, growth, and development. To prevent deficiency, many countries fortify table salt with iodine, creating iodized salt — the primary source of iodine in most modern diets. The U.S. began salt iodization in 1924, and this simple public health measure continues to protect millions of people worldwide from thyroid disorders.However, natural food sources of iodine vary greatly depending on the amount of iodine in local soil and water. Seafood, such as fish and shrimp, are rich in iodine, but people living far from the ocean often have limited access to these sources. 

Iodine-deficiency hypothyroidism is now uncommon in countries that iodize salt, so in places such as the United States and Western Europe most cases are autoimmune  — Hashimoto's thyroiditis — rather than dietary. It remains a significant problem where iodized salt is not reliably available, especially in Central and Eastern sub-Saharan Africa, South Asia (India, Nepal, Pakistan), and inland or mountainous regions with iodine-poor soils and highly local diets. Globally, hundreds of millions consume too little iodine, and a smaller but still substantial subset develop goiter or hypothyroidism as a result.

When the body lacks iodine, the thyroid gland cannot make enough T₃ and T₄. In response, the pituitary gland releases extra thyroid-stimulating hormone (TSH), which overstimulates the thyroid and causes it to enlarge — forming a goiter, an enlargement of the thyroid gland.

Figure \(\PageIndex{1}\): Goiter. A goiter is the abnormal enlargement of the thyroid gland. (Image credit: "Goiter"(opens in new window) by Almazi is licensed under CC BY 4.0)

Furthermore, iodine deficiency can have other serious effects on health and development. In children, it can cause stunted growth and delayed physical and mental development. In fact, iodine deficiency is the leading preventable cause of intellectual disability worldwide. Adults may experience reduced fertility and complications during pregnancy, and infants born to iodine-deficient mothers face a higher risk of mortality. One of its most severe outcomes is congenital hypothyroidism (also called cretinism), which results from maternal iodine deficiency during pregnancy and can lead to lifelong cognitive impairment, short stature, and sometimes hearing or speech difficulties.

Other Types of Hypothyroidism

Hypothyroidism is a condition in which the thyroid gland produces insufficient thyroid hormone. In regions where iodine deficiency is rare, the most common cause of hypothyroidism is Hashimoto’s thyroiditis, an autoimmune disorder in which the immune system attacks the thyroid, leading to chronic inflammation and gradual loss of function. Other causes include surgical removal of the thyroid and certain medications that interfere with hormone synthesis.

When thyroid activity decreases, metabolic processes slow down. Typical symptoms include fatigue, weight gain despite normal eating, cold intolerance, dry skin and hair, constipation, slow heart rate, and sometimes depression or slowed thinking. In people who menstruate, periods may become heavy or irregular. Laboratory findings usually show low T₃ and T₄ levels with elevated TSH, as the pituitary gland increases stimulation in an attempt to boost thyroid output. This persistent stimulation can cause the thyroid to enlarge, forming a goiter.

Figure \(\PageIndex{2}\): Hypothyroidism can affect nearly every organ system because thyroid hormones regulate metabolism, growth, and energy balance. The image highlights common systemic effects: slowed brain function; fatigue and cold intolerance due to reduced metabolism; slowed heart rate and poor circulation; constipation from reduced digestive motility; muscle weakness; and menstrual irregularities or infertility. In children, hypothyroidism can impair growth and development, while in adults it leads to generalized slowing of body processes. (Image credit: “Human endocrine system with target organs” by Mikael Häggström, licensed under CC0/public domain via Wikimedia Commons.)
 

 

 

Hyperthyroidism

Hyperthyroidism is the opposite of hypothyroidism: the thyroid produces and releases too much hormone. The most common cause is Graves disease, another autoimmune disorder in which antibodies overstimulate the thyroid so it enlarges and secretes excess T₃ and T₄. Other causes include autonomously functioning thyroid nodules or, less commonly, a pituitary tumor that produces too much TSH. Because metabolism is accelerated, patients may show unintentional weight loss, heat intolerance, excessive sweating, nervousness or irritability, insomnia, rapid or irregular heartbeat, and more frequent bowel movements. In Graves disease, inflammation behind the eyes can cause exophthalmos (bulging eyes). Laboratory findings typically show elevated T₃ and T₄ with low TSH, because the pituitary tries to turn down the signal.

 

Both thyroid disorders are important clinically because their symptoms can mimic depression, anxiety, menopause, or normal aging. In summary:

• Hypothyroidism slows body processes,
• Hyperthyroidism speeds them up.

Figure \(\PageIndex{3}\): Hyperthyroidism. Amongst the typical signs and symptoms — besides exophthalmos — are: anxiety, tremor, tachycardia, heat intolerance, weight loss, goiter.