Hypothyroidism is a common endocrine disorder resulting from deficiency of thyroid hormone. Iodine deficiency remains the most common cause of hypothyroidism worldwide.

The most common symptoms in adults are fatigue, lethargy, cold intolerance, weight gain, constipation, depression, change in voice, and dry skin, but clinical presentation can differ with age and sex, among other factors.

Hypothyroidism may be:

Primary hypothyroidism

Primary hypothyroidism is due to decreased secretion of T4 and T3 from the thyroid.

Serum T4 and T3 levels are low, and thyroid-stimulating hormone (TSH) is increased.

Secondary hypothyroidism

Secondary hypothyroidism occurs when the hypothalamus produces insufficient thyrotropin-releasing hormone (TRH) or the pituitary produces insufficient TSH.

In secondary hypothyroidism, free T4 and serum TSH are low (sometimes TSH is normal but with decreased bioactivity).

Subclinical hypothyroidism

Sub-clinical hypothyroidism is a state of usually asymptomatic, mild thyroid failure, with normal levels of T4 and T3, and minimal elevation of TSH.

Management of Primary hypothyroidism

Offer levothyroxine as first-line treatment for adults, children and young people with primary hypothyroidism.

Consider starting levothyroxine at a dosage of 1.6 micrograms per kilogram of body weight per day (rounded to the nearest 25 micrograms) for adults under 65 with primary hypothyroidism and no history of cardiovascular disease.

Consider starting levothyroxine at a dosage of 25 to 50 micrograms per day with titration for adults aged 65 and over and adults with a history of cardiovascular disease.

Aim to maintain TSH levels within the reference range when treating primary hypothyroidism with levothyroxine. 

Management of Subclinical hypothyroidism

When discussing whether or not to start treatment for subclinical hypothyroidism, take into account features that might suggest underlying thyroid disease, such as symptoms of hypothyroidism, previous radioactive iodine treatment or thyroid surgery, or raised levels of thyroid autoantibodies.

Adults

Consider levothyroxine for adults with subclinical hypothyroidism who have a TSH of 10 mlU/litre or higher on 2 separate occasions 3 months apart.

Consider a 6-month trial of levothyroxine for adults under 65 with subclinical hypothyroidism who have:

• a TSH above the reference range but lower than 10 mlU/litre on 2 separate occasions 3 months apart, and
• symptoms of hypothyroidism.