Hypothyroidism

(Overt) hypothyroidism refers to decreased production of T3 and T4 by the thyroid gland. It’s a common condition. Chronic hypothyroidism is almost always due to Hashimoto thyroiditis.

Subclinical hypothyroidism is very common, but despite being subclinical it may be associated with adverse outcomes on cardiovascular disease. Also, most patients with subclinical hypothyroidism progress into overt hypothyroidism.

Etiology

• Primary hypothyroidism
  • Thyroiditis
    • Hashimoto thyroiditis
    • De Quervain thyroiditis
    • Postpartum thyroiditis
  • Following surgical thyroid removal or radioiodine treatment
  • Iodine deficiency
  • Drugs
• Secondary hypothyroidism
  • TSH insufficiency
• Tertiary hypothyroidism
  • TRH insufficiency

Thyroiditis, especially Hashimoto thyroiditis, is the most common cause of hypothyroidism. Secondary and tertiary hyperthyroidism are rare.

Clinical features

Hypothyroidism can also cause many symptoms, most of which are opposite of those of thyrotoxicosis:

• Weakness
• Memory problems
• Depression
• Dry skin
• Myxoedema
• Hair loss
• Constipation
• Weight gain
• Cold intolerance
• Bradycardia
• Anaemia
• Menstruation problems
• Hypercholesterolaemia -> accelerated atherosclerosis

Diagnosis and evaluation

TSH is the most important biomarker in the evaluation of hypothyroidism as it’s much more sensitive than T3 and T4. T3 and T4 are only measured to diagnose subclinical hypothyroidism, the stage before overt hypothyroidism develops.

	TSH	Free T4	Free T3
Euthyroidism	Normal	Normal	Normal
Subclinical hypothyroidism	Increased	Normal	Normal
Overt hypothyroidism	Increased	Decreased	Decreased

To find the etiology, measurement of anti-thyroid antibodies (anti-TPO and anti-Tg), ultrasound, and FNAB are used.

Anti-TPO and elevated in autoimmune thyroiditis. Anti-thyroglobulin may also be used in autoimmune thyroiditis, but is less sensitive and specific than anti-TPO, so anti-TPO is used instead.

Treatment

The treatment of hypothyroidism is thyroid hormone replacement with L-thyroxine. The dose must be adjusted until the TSH is in the normal range. The beginning daily dose is 50 – 100 µg, but in elderly, who often have ischaemic heart disease and are therefore predisposed to arrhythmias or MI, the beginning dose must be lower.

The thyroxine dose must be increased during pregnancy as the requirement is higher in this stage.

The decision of whether to treat subclinical hypothyroidism is less clear. In general, thyroxine replacement is indicated for subclinical hypothyroidism in case of:

• Very high TSH or autoantibodies
• Infertility
• Depression