Hypothyroidism

It is a deficiency of thyroid hormones. It is a chronic condition seen more commonly in females.

Types of hypothyroidism

Type: Primary

• Features: Disorder of the thyroid gland causing decreased synthesis or secretion of thyroid hormones (TH)
• Causes: Autoimmune thyroiditis, postpartum and silent thyroiditis, iodine deficiency or excess, thyroidectomy, radioactive iodine, external radiotherapy, drugs like amiodarone, lithium, interferon alpha, Sunitinib; congenital in thyroid agenesis or dysgenesis

Type: Secondary (central)

• Features: Results from deficiency of TSH
• Causes: Pituitary adenomas, head trauma, pituitary apoplexy, Sheehan’s syndrome, surgery, radiotherapy, genetic, infiltrative disorders

Type: Tertiary (central)

• Features: Results from deficiency of TRH (thyrotropin releasing hormone)
• Causes: Head trauma, surgery, radiotherapy, infiltrative disorders, genetic

Type: Peripheral

• Features: Aberrant expression of deiodinase 3 enzyme, increased reverse T3
• Causes: Malignancies, hepatic haemangiomatosis, GIST, tissue resistance to TH

Clinical features include fatigue, lethargy, cold intolerance, weight gain, constipation, change in voice and dry skin. Many patients are asymptomatic. Hypothyroidism affects all organ systems, with profound effects on the cardiovascular system and metabolism. It causes dyslipidemia with increased LDL and total cholesterol, endothelial dysfunction, low voltage ECG, hypertension, bradycardia, diastolic dysfunction, hyperhomocysteinemia, neuropathy, delayed relaxation phase of deep tendon reflexes, impaired cognition, depression, dementia, carpal tunnel syndrome, non-alcoholic fatty liver, infertility, increased prolactin, glucose intolerance, gastroparesis, myopathy, CPK elevation, yellow palms, loss of lateral eyebrows, hyponatremia and reduced GFR. Turner syndrome carries a higher risk of hypothyroidism.

Congenital hypothyroidism presents with jaundice, lethargy, constipation, hypothermia, bradycardia, omphalocele, growth retardation and cretinism.

Hormone levels in hypothyroidism

Free T4 levels may be elevated from heparin. Increased free T4 with normal TSH is seen in overtreatment with levothyroxine. Elevated TSH with normal free T4 may be seen in no-compliance with therapy. Patients can have normal TSH levels but still have symptoms due to concurrent disorders like Addison’s disease, pernicious anemia, rheumatoid arthritis and euthyroid sick syndrome. MRI brain is done to evaluate central hypothyroidism.

Myxedema coma is seen in severe, untreated hypothyroidism and presents with altered mental status, hypothermia, progressive lethargy, bradycardia, shock and can eventually result in multiple organ dysfunction syndrome and death.

Treatment of hypothyroidism is with oral levothyroxine. Treatment is recommended when the TSH level is >10 mIU/L, even if the free T4 level is normal. Increase in dose is needed during pregnancy. Osteoporosis and atrial fibrillation can result from overtreatment with levothyroxine. Calcium or iron containing supplements, proton pump inhibitors, aluminium containing antacids and orlistat can interfere with the absorption of levothyroxine. Drugs like phenobarbital, warfarin, carbamazepine, rifampin and oral hypoglycemics increase the metabolism of thyroxine and drug dose may need to be increased. While furosemide, mefenamic acid and salicylates decrease the metabolism of levothyroxine. Celiac disease and H.pylori gastritis can interfere with absorption. Therapy for congenital hypothyroidism should be initiated in the first two weeks of life. Myxedema coma is treated with intravenous levothyroxine , hydrocortisone and supportive therapy.