Thyroid Disorders

The thyroid is a small gland at the front of the neck that produces thyroxine (T4) and triiodothyronine (T3), the hormones setting the pace of metabolism, heat production, heart rate, gut motility and neurological function. Output is governed by TSH from the pituitary in a feedback loop, which is why TSH — not T4 — is the most sensitive early indicator of a problem. Thyroid disorders divide into four broad groups. Hypothyroidism, where too little hormone is produced, is the most common and is usually caused by Hashimoto's thyroiditis. Hyperthyroidism, where too much is produced, is most often Graves' disease, sometimes a toxic nodule or transient thyroiditis. Structural disease includes nodules — common, and overwhelmingly benign — and goitre. Thyroid cancer is uncommon and generally has an excellent prognosis. Symptoms are notoriously unreliable on their own: fatigue, weight change and low mood are shared with a dozen other conditions, which is why so many cases are found incidentally on bloods. Diagnosis is straightforward once tested, and treatment is effective — levothyroxine for underactivity, antithyroid drugs, radioiodine or surgery for overactivity. Micronutrients matter at the margins: iodine, selenium and iron are all needed for normal hormone synthesis and conversion, but supplementing beyond need does not improve a healthy thyroid and excess iodine can cause harm.

• •Underactive: fatigue, cold intolerance, weight gain, constipation, dry skin, low mood
• •Underactive: hair thinning, heavy periods, muscle cramps, slowed heart rate
• •Overactive: weight loss despite good appetite, heat intolerance, sweating
• •Overactive: palpitations, tremor, anxiety, insomnia, frequent loose stools
• •Structural: a lump or swelling in the neck, difficulty swallowing, hoarseness
• •Eye changes — bulging, grittiness or double vision, suggesting Graves' orbitopathy

• •Hashimoto's thyroiditis — the leading cause of hypothyroidism where iodine is adequate
• •Graves' disease — the leading cause of hyperthyroidism
• •Iodine deficiency, still the commonest cause worldwide
• •Excess iodine, including from amiodarone, contrast media or kelp
• •Postpartum thyroiditis and viral (subacute) thyroiditis
• •Thyroid surgery or radioiodine treatment
• •Medications including lithium, interferon and checkpoint inhibitors
• •Congenital thyroid disorders and pituitary disease

Most thyroid dysfunction is autoimmune: a loss of immune tolerance leading either to destruction of the gland (Hashimoto's) or to stimulating antibodies that drive it beyond feedback control (Graves'). Beneath that sit genetic susceptibility, female sex, and triggers such as pregnancy, iodine shifts, smoking and certain drugs. A distinct second pathway is nutritional — iodine deficiency limits hormone synthesis directly, while low selenium impairs T4 to T3 conversion and low iron reduces thyroid peroxidase activity.