Write a short note on hypothyroidism, Hashimoto's thyroiditis and myxoedema coma.

Definition & Causes

Hypothyroidism is the clinical state resulting from deficient thyroid hormone action. It affects around 1–2% of the population, is 5–10 times commoner in women, and rises in prevalence with age.

Hashimoto's Thyroiditis

An autoimmune disorder with lymphocytic infiltration and germinal centre formation, destroying thyroid tissue. Anti-thyroid peroxidase (anti-TPO) antibodies are positive in over 90%, and anti-thyroglobulin in many. It presents with a firm, non-tender, diffuse goitre and progressive hypothyroidism, though a transient thyrotoxic phase ("Hashitoxicosis") may occur early as stored hormone is released. It is associated with other autoimmune disease (type 1 diabetes, Addison's, vitiligo, pernicious anaemia) and carries a small increased risk of thyroid lymphoma — suspect this if the gland enlarges rapidly.

Clinical Features

• General: fatigue, cold intolerance, weight gain despite reduced appetite, hoarse voice, somnolence.
• Skin and appendages: dry coarse skin, non-pitting periorbital puffiness (myxoedema), brittle hair, loss of the outer third of the eyebrows (madarosis), carotenaemia with yellowish palms.
• Cardiovascular: bradycardia, hypertension (diastolic), pericardial effusion, cardiomegaly, hypercholesterolaemia with accelerated atherosclerosis.
• Neuromuscular: delayed relaxation phase of the ankle jerk — a classic bedside sign, myalgia, proximal myopathy with raised CK, carpal tunnel syndrome, paraesthesiae, cerebellar ataxia.
• Gastrointestinal: constipation, ileus, ascites.
• Reproductive: menorrhagia, infertility, galactorrhoea (raised TRH stimulates prolactin), erectile dysfunction.
• Neuropsychiatric: poor memory and concentration, depression, and in severe cases "myxoedema madness" with psychosis.
• Children: growth failure and, in congenital hypothyroidism, irreversible intellectual disability — the reason for newborn screening.

Investigations

• TSH — the single best test. RAISED in primary hypothyroidism with low free T4.
• Subclinical hypothyroidism: raised TSH with normal free T4.
• Secondary hypothyroidism: low free T4 with low or inappropriately normal TSH — TSH alone will miss this, so measure free T4 when pituitary disease is suspected.
• Anti-TPO antibodies — confirm autoimmune aetiology and predict progression from subclinical to overt disease.
• Supporting: hypercholesterolaemia, macrocytic anaemia, raised creatine kinase, hyponatraemia; ECG showing bradycardia and low-voltage complexes; ultrasound for nodular disease.

Management

• Levothyroxine (T4) — the treatment of choice, given once daily on an empty stomach, 30–60 minutes before breakfast, and separated from calcium, iron, proton pump inhibitors and soy, which impair absorption.
• Start at a low dose and titrate slowly in the elderly and in ischaemic heart disease — abrupt full replacement can precipitate angina, infarction or arrhythmia.
• Recheck TSH after 6–8 weeks and adjust; once stable, monitor annually. Titrate to a normal TSH in primary disease, but to free T4 in secondary disease, where TSH is uninformative.
• Pregnancy: requirements increase by 25–50%; check TSH each trimester and treat promptly, since maternal hypothyroidism impairs fetal neurodevelopment.
• Subclinical hypothyroidism: treat if TSH is markedly raised, if anti-TPO positive with symptoms, in pregnancy or when planning conception, or where there is goitre or dyslipidaemia; otherwise monitor.
• Combination T4/T3 therapy is not routinely recommended.

Myxoedema Coma

The decompensated end-stage of severe untreated hypothyroidism — a rare emergency with mortality of 20–50%. Typically an elderly woman in winter, with undiagnosed or untreated hypothyroidism.

Precipitants: cold exposure, infection, stroke, myocardial infarction, trauma, surgery, and sedatives, opioids or anaesthetics.

Features: hypothermia (often without shivering), depressed consciousness progressing to coma, bradycardia and hypotension, hypoventilation with hypercapnia and hypoxia, hyponatraemia, hypoglycaemia, seizures, ileus and urinary retention, with the physical stigmata of hypothyroidism.

Management — intensive care, treat before confirmation:

1. Intravenous levothyroxine (with or without liothyronine/T3) — loading dose then maintenance.

2. Intravenous hydrocortisone BEFORE thyroid hormone — coexisting adrenal insufficiency is common, and giving thyroid hormone alone can precipitate an adrenal crisis.

3. Passive rewarming — active external rewarming causes peripheral vasodilatation and cardiovascular collapse.

4. Ventilatory support for hypercapnic respiratory failure; cautious fluids; correct hyponatraemia slowly and treat hypoglycaemia.

5. Identify and treat the precipitant, especially infection — fever may be absent, so have a low threshold for antibiotics.

6. Avoid sedatives, and use all drugs cautiously as metabolism is markedly slowed.