Adding iodine to table salt quietly prevented a common hormone disease
Around a billion people worldwide may lack enough iodine, the element the thyroid needs to build its hormones. Shortage is the leading global cause of an underactive thyroid, which brings tiredness, cold intolerance and weight gain. Iodised salt has prevented it in many populations; where iodine is plentiful, an autoimmune attack is the usual culprit.
The thyroid is the body's only source of thyroid hormone, assembling it from iodine and the amino acid tyrosine. It mainly releases thyroxine, or T4, which other organs convert into the more active T3 using a selenium-dependent enzyme. T3 enters cell nuclei and switches on particular genes. Production is governed by a chain of command: the hypothalamus releases TRH, which prompts the pituitary to release TSH, which drives the thyroid, while circulating thyroxine damps the whole chain down.
Failure at the gland itself, primary hypothyroidism, is roughly a thousand times commoner than failure higher up the chain. In iodine-sufficient countries the leading cause is Hashimoto's thyroiditis, in which immune cells and antibodies target the gland; it often travels with other autoimmune conditions such as type 1 diabetes and coeliac disease. Other causes include thyroid surgery, radioactive iodine treatment and some medicines. About 5 percent of women develop thyroiditis after giving birth, and 20 to 40 percent of those stay permanently underactive.
Symptoms are often mild or absent and easily blamed on something else, including menopause in middle-aged women. A telltale sign is a slow relaxation phase after the ankle-jerk reflex. In the US, overt disease affects roughly 0.3 to 0.4 percent of people, while a milder subclinical form, with high TSH but normal thyroxine, may affect 4.3 to 8.5 percent. It is more frequent in women and in people over 60. At the extreme lies myxedema coma, a rare emergency marked by low temperature without shivering, confusion and slowed breathing.
The stakes are highest early in life. Untreated deficiency in pregnancy can impair a baby's growth and intellect, and untreated severe cases in infants can lower IQ by 6 to 15 points. A blood test for TSH is the preferred first step. Replacement with levothyroxine, adjusted until TSH normalises, is the standard treatment and is considered safe in pregnancy. Dogs develop the condition too, and cats and horses occasionally.
Source: Hypothyroidism
