Last updated:
March 19, 2026
Hypothyroidism is a deficiency of the thyroid hormones T3 and T4. The presentation of hypothyroidism can be subtle and non-specific since almost all cell nuclei have thyroid receptors, which affect various metabolic processes. Prognosis is good if treated, but it can be disastrous if left untreated.
Hypothyroidism is common, affecting 4 per 1000 individuals per year.
The most common cause of hypothyroidism is Hashimoto’s thyroiditis.
Classification of Hypothyroidism
| Classification | Description | Causes | Lab findings |
|---|---|---|---|
| Primary hypothyroidism | Decreased production of thyroid hormone from the thyroid gland | Hashimoto thyroiditis, drug-induced hypothyroidism, thyroiditis, iatrogenic (post-RAI ablation), iodine deficiency, congenital (dysplasia or aplasia) | High TSH, low T3 and T4 |
| Primary autoimmune | Primary atrophic hypothyroidism (due to diffuse lymphocytic infiltration), Hashimoto’s thyroiditis | ||
| Drug-induced | Antithyroid drugs, amiodarone, lithium, iodine | ||
| Iatrogenic | Post-thyroidectomy, post-radioiodine ablation | ||
| Subacute thyroiditis | Temporary hypothyroidism after a hypothyroid state | ||
| Iodine deficiency | The most common cause worldwide | ||
| Secondary hypothyroidism | Decreased thyroid hormone due to decreased TSH (hypopituitarism). Very rare | Tumor, mass effect | Low TSH, low T3 and T4 |
| Tertiary hypothyroidism | Decreased thyroid hormone due to decreased TRH. Very rare | Tumor, mass effect | Low TSH, low T3 and T4 |
Signs and symptoms of hypothyroidism
| Classification | Signs and symptoms |
|---|---|
| Reduced metabolic rate | Fatigue, cold intolerance, weight gain despite reduced appetite, constipation, bradycardia, decreased deep tendon reflexes, hair loss |
| Increased stimulation | Painless goitre |
| Increased TRH | Hyperprolactinemia, menorrhagia, amenorrhoea, galactorrhoea |
| Deposition of glycosaminoglycans | Myxoedema (non-pitting oedema), hoarseness, pretibial and periorbital oedema |
| Psychiatric | Depression, pseudodementia (older individuals), impaired memory or cognition |
- Associated conditions
- Type 1 diabetes mellitus
- Addison’s disease
- Turner’s and Down’s syndrome
- Cystic fibrosis
- Primary biliary cholangitis
- Ovarian hyperstimulation
- POEMS syndrome (Polyneuropathy, organomegaly, endocrinopathy, M-protein band)
- Dyshormonogenesis, e.g Pendred’s syndrome
- Signs and symptoms
- Fatigue (most common complaint)
- Cold intolerance
- Weight gain
- Reduced appetite
- Constipation
- Hair loss
- Painless goitre
- Menstrual cycle abnormalities
- Pretibial and periorbital oedema
- Depression
- Pseudodementia
- Reduced memory
- Physical examination (BRADYCARDIC)
- Bradycardia
- Reflexes reduced
- Ataxia (cerebellar)
- Dry-thin hair or skin
- Yawning, drowsy or coma
- Cold peripheries due to reduced temperature
- Ascites +/- non-pitting oedema +/- pericarial or pleural effusion
- Round puffy face or obese
- Defeated demeanour
- Immobile +/- Ileus
- Congestive heart failure
- Differentials
- Pituitary adenoma
- Cushing’s syndrome
- Myxedema coma
- Investigations
- Complete blood count to rule out anaemia as a cause of fatigue
- Serum TSH levels
- Elevated TSH is seen in primary hypothyroidism
- Serum T3 and T4
- Low T4 and high TSH confirm primary hyperthyroidism
- Normal T4 and high TSH indicate subclinical hyperthyroidism
- Serum Thyroglobulin (Tg) and anti-thyroglobulin Antibody (TgAb) tests to identify autoimmune thyroid disease
- Elevated in Hashimoto’s thyroiditis
- Serum Thyrotropin Receptor Antibodies (TRAb) test to rule out Graves’ disease
- Ultrasound or nuclear scan if a goitre or nodule is present
- Treatment
- Daily Levothyroxine (T4)
- Start at a lower dose in elderly patients or those with ischaemic heart disease (may precipitate angina or MI), e.g. 25 mcg
- Other patients can be started at 50 – 100 mcg
- Review at 12 weeks
- Adjust 6-weekly by clinical state and normalise TSH (0.5 – 2.5 mU/L)
- Increase dose by 25 – 50 ug in pregnant women and monitor TSH (while aiming for lower values)
- Daily Levothyroxine (T4)
- Side effects of levothyroxine
- Hyperthyroidism (over-treatment)
- Reduced bone mineral density
- Worsening of angina
- Atrial fibrillation
- Levothyroxine also interacts with iron absorption and should be given at least 2 hours apart
- Complications of hypothyroidism
- Hypercholesterolemia
- Myxedema coma
- Complications of hypothyroidism in pregnancy
- Eclampsia
- Anaemia
- Prematurity
- Low birthweight
- Stillbirth
- Post-partum hemorrhage