Neobook — clinical reference

ICD-10 Code: E03.0-5/E03.8-9

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.

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.

Standard Treatment Guidelines and Essential Medicines List for South Africa, Hospital Level, Adults 2024 Edition

Hypothyroidism