Hypothyroidism

Related Subjects: |Thyrotoxicosis and Hyperthyroidism |Thyroid Storm - Thyrotoxic crisis |Graves Disease (Thyrotoxicosis) |Amiodarone and Thyroid disease |Thyroid Surgery (Thyroidectomy) |Hypothyroidism |Hashimoto's thyroiditis |DeQuervain's thyroiditis |Subacute Thyroiditis |Thyroid nodule |Congenital Hypothyroidism |Thyroid Function Tests and antibodies |Post partum thyroiditis |Sick Euthyroid Syndrome |Thyroid Exam (OSCE) |Thyroid Gland anatomy and Physiology |Thyroid Cancer

Start with a lower dose of Thyroxine in the elderly and in those with angina or heart failure

Introduction

• Hypothyroidism is a disease in which there is inadequate production of thyroxine for normal physiological functions.
• The thyroid gland requires iodine to make thyroxine. The gland produces both T4 and T3 molecules but T3 is the most active form.
• Generally the T4 level is low but in clinical care, the best test for hypothyroidism is a raised TSH.
• This has been found to be an exquisitely sensitive indicator of thyroid status.
• A normal TSH result suggests adequate thyroid hormone replacement and euthyroidism.

Aetiology

• Primary hypothyroidism (95% of cases) occurs when the thyroid gland is unable to produce T3/T4 because of iodine deficiency or an abnormality within the gland itself
• Secondary "pituitary" hypothyroidism is the result of insufficient production of bioactive TSH release because of a pituitary or hypothalamic disorder

Causes of primary hypothyroidism

• There are a variety of these from surgical removal or radiotherapy of the gland to iodine deficiency and inflammatory conditions
• Iodine deficiency: iodine deficiency is the most common cause of hypothyroidism. Seen worldwide.
• Autoimmune thyroiditis such as Hashimoto's or atrophic thyroiditis where there is the destruction of thyroid follicular cells by lymphocytes. It may be associated with a goitre (Hashimoto's thyroiditis) or occur without a goitre (atrophic thyroiditis or primary myxoedema)
• Post-ablative therapy or surgery: after surgery, radioiodine therapy, or external radiotherapy
• Drugs: Anti-thyroid drugs for Graves disease (such as carbimazole and propylthiouracil) will develop hypothyroidism 10-20 years later. Treatment with several other drugs, such as iodine (including kelp supplements), amiodarone, lithium, interferons, thalidomide, and rifampicin
• Transient thyroiditis: Subacute (de Quervain's) thyroiditis with painful swelling of the thyroid gland thought to be caused by a viral infection
• Postpartum thyroiditis (PPT) is the development of thyrotoxicosis, hypothyroidism, or thyrotoxicosis followed by hypothyroidism within a year of giving birth, in women who were euthyroid prior to pregnancy
• Thyroid infiltrative disorders: amyloidosis, sarcoidosis, haemochromatosis, tuberculosis, and scleroderma
• Congenital hypothyroidism: absence or underdevelopment of the thyroid gland (60%), an ectopic hypoplastic gland (30%), or the absence of enzymes required for thyroid hormone synthesis and iodide transfer

Clinical

• Rare but must know about Myxoedema coma (See topic)
• Clinically hypothyroid, Tiredness, lethargy, weight gain
• Depression, constipation, cold intolerance
• Poor memory, slowness, depression, dry and cold skin
• Heavy or infrequent periods, psychosis
• Ataxia and slowness of movement
• Dry thin hair, hypothermia, bradycardia
• heart failure, pericardial effusion, weight gain and obesity
• "hung up" slow relaxing reflexes and Myotonia and proximal myopathy
• Children develop cretinism if congenital hypothyroid

Before and after starting Thyroxine

Some may have a Goitre

Differential

• Hypopituitary

Investigation

• FBC: anaemia, macrocytes
• CXR: pleural effusion, pericardial effusion
• ECG: bradycardia, low voltage
• TFTS: high TSH low T4 and T3 unless pituitary disease and T4 and TSH usually low/normal or even mildly elevated
• Thyroid ultrasound if enlarged or nodules

Classification

• TSH high usually > 10 , T4 is below normal range: Treats as Primary hypothyroid
• TSH high, T4 is normal: Subclinical hypothyroid. Treat if TSH > 10 or symptoms or peroxidase positive
• TSH high, T4 above normal range. Endocrine referral
• TSH normal: Normal Euthyroid patient
• TSH low Consider if hyperthyroid or Primary hypothyroid

ECG in hypothyrodism

Treatment Algorithm

Management

• Primary hypothyroidism usually has a good prognosis and most people will recover full health following adequate thyroid hormone replacement with levothyroxine (LT4), which is usually needed for life
• Commence Thyroxine 50 mcg od and increase to 100-150 mcg OD. Use 25 mcg dose in elderly or those at risk of IHD
• There is a small number of individuals who require T3 rather than T4

References

• Hypothyroidism: An Update