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
- 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.
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