E03.0-5/E03.8-9
DESCRIPTION
Causes
Common causes of primary hypothyroidism are:
- chronic autoimmune thyroiditis,
- post-surgery, and
- post radio-active iodine.
Secondary hypothyroidism (less than 1% of cases) may be due to any cause of anterior hypopituitarism.
Investigations
Thyroid stimulating hormone (TSH) and thyroxine (T₄) initially. In primary hypothyroidism TSH is elevated and T₄ is low. If TSH is normal or slightly elevated and T₄ is low this suggests hypopituitarism: take blood for cortisol and ACTH, give hydrocortisone replacement before starting levothyroxine and investigate for causes of hypopituitarism.
MEDICINE TREATMENT
- Levothyroxine, oral, 100 mcg daily.
- If there is a risk of ischaemic heart disease, start at 25 mcg daily and increase by 25 mcg every 4 weeks.
Check TSH and T₄ after 2–3 months and adjust dose if required.
TSH levels will take several weeks to stabilise. Once stable check T₄ and TSH annually.
Hypothyroidism in pregnancy
About 60% of hypothyroid pregnant women need an increase in levothyroxine therapy in the second and third trimesters. Because T₄ takes a long time to reach steady state and 1st trimester hypothyroidism is undesirable for the fetus, for patients with borderline control (TSH>1.2mU/L) it is advisable to increase the pre-pregnancy dose by 30%. Check TSH monthly and increase levothyroxine doses to keep serum TSH levels low normal and free T₄ levels in the high-normal range. After delivery, revert to pre-conception doses.
Note: TSH and T₄ reference range is trimester-specific.