Comparing symptoms
Common causes of each
Hypothyroidism: Hashimoto's thyroiditis (the most common cause worldwide — an autoimmune destruction of the thyroid) accounts for the majority of cases in iodine-sufficient countries. Other causes include prior thyroid surgery or radioactive iodine treatment, medications (notably amiodarone and lithium), and iodine deficiency globally.
Hyperthyroidism: Graves' disease — an autoimmune condition where antibodies stimulate TSH receptors — causes around 70–80% of cases. Toxic multinodular goitre and a single toxic adenoma account for most of the rest. Medications (excess thyroid hormone, amiodarone) and thyroiditis (inflammation releasing stored hormone) are less common causes.
Blood tests and what they show
TSH (thyroid-stimulating hormone) is the primary screening test. The pituitary gland produces more TSH when thyroid output is low (hypothyroidism) and suppresses TSH when output is high (hyperthyroidism). Free T4 and free T3 confirm and quantify the abnormality.
- High TSH + Low Free T4 = Primary hypothyroidism
- Low TSH + High Free T4 and/or Free T3 = Hyperthyroidism
- High TSH + Normal Free T4 = Subclinical hypothyroidism
- Low TSH + Normal Free T4 = Subclinical hyperthyroidism
- Thyroid antibodies (TPO, TG, TSH-receptor) identify the underlying autoimmune cause
Treatment
Hypothyroidism is treated with levothyroxine (synthetic T4), taken daily. Dose is titrated to bring TSH within the normal range. Most people feel significantly better within 4–8 weeks of starting. Thyroid function should be rechecked 6–12 weeks after any dose change.
Hyperthyroidism is managed with antithyroid drugs (carbimazole or propylthiouracil), radioactive iodine ablation, or thyroidectomy — each with specific indications. Beta-blockers control symptoms (palpitations, tremor) while definitive treatment takes effect.