Thyroid Disorders
Thyroid testing and hypothyroidism
TSH is the first-line test for primary thyroid dysfunction; free T4 is added when TSH is abnormal or central disease is suspected. Primary hypothyroidism shows high TSH and low free T4; subclinical hypothyroidism shows high TSH with normal free T4. Central hypothyroidism shows low free T4 with low or inappropriately normal TSH. Levothyroxine is dosed around 1.6 mcg/kg/day in healthy adults, started lower in older patients or those with coronary disease. Recheck TSH 6–8 weeks after dose changes. Subclinical hypothyroidism is usually treated when TSH is >10 mIU/L, symptoms are present, or the patient is pregnant.
Hyperthyroidism and nodules
Graves disease causes low TSH, high free T4, diffuse goiter, and positive TSH receptor antibodies. Toxic nodules and thyroiditis also suppress TSH, but thyroiditis often has low radioiodine uptake and a tender gland. Beta-blockers control symptoms; thionamides inhibit hormone synthesis. In pregnancy, PTU is preferred in the first trimester and methimazole thereafter. Thyroid storm is diagnosed clinically and treated with beta-blockade, a thionamide, iodine after the thionamide, and glucocorticoids. Thyroid nodules are evaluated with TSH and ultrasound; fine-needle aspiration is based on size and sonographic suspicion.
Traps and worked example
Biotin can distort immunoassays, and amiodarone can cause both excess iodine-related hyperthyroidism and destructive thyroiditis. A 34-year-old woman has palpitations, weight loss, TSH 0.01 mIU/L, free T4 3.2 ng/dL, and a diffuse goiter with TSH receptor antibodies; this is Graves disease, not subacute thyroiditis. Rule of thumb: in pregnancy, use trimester-specific TSH ranges and do not rely on symptoms alone.