Central hypothyroidism: an underactive thyroid caused by the pituitary

The thyroid is controlled from the brain. The pituitary gland, a small gland at the base of the brain, releases thyroid-stimulating hormone (TSH). The Pituitary Foundation explains that TSH acts on the thyroid gland in the neck, which makes thyroxine, important for the body’s metabolism. In the common form of underactive thyroid, the thyroid itself is damaged, usually by autoimmune Hashimoto’s disease according to the NHS, and the pituitary responds by producing more TSH.

In central hypothyroidism the problem lies upstream. The pituitary, or less often the hypothalamus above it, does not send enough of a signal, so the thyroid is underactive because it has not been told to work. The British Thyroid Foundation lists a malfunction of the pituitary gland as one of the causes of hypothyroidism. Doctors sometimes use the terms secondary hypothyroidism for a pituitary cause and tertiary for a hypothalamic one, and central hypothyroidism covers both.

The Pituitary Foundation lists the causes of hypopituitarism, which are also the causes of central hypothyroidism when TSH is affected. The most common is a benign (non-cancerous) pituitary tumour pressing on the gland.

• A benign tumour of the pituitary gland, the most common cause.

• Surgery or radiotherapy used to treat a pituitary tumour.

• Cysts in or near the pituitary, such as Rathke’s cleft cysts, and benign tumours around the gland such as craniopharyngiomas and meningiomas.

• Serious head injuries.

• Reduced blood flow to the gland or bleeding into it, including Sheehan’s syndrome after severe blood loss in childbirth and pituitary apoplexy.

• Inflammation of the gland (hypophysitis), most commonly seen during pregnancy in its lymphocytic form.

• Genetic conditions affecting how the gland develops.

• Infections such as meningitis, and conditions such as sarcoidosis.

• Long-term use of some medicines such as glucocorticoids, and some cancer treatments.

The Pituitary Foundation lists the symptoms of TSH deficiency as tiredness, weight gain, dry skin, constipation and sensitivity to cold or finding it difficult to stay warm. These overlap with those of the ordinary underactive thyroid listed by the NHS, which include fatigue, feeling cold, weight gain, constipation, difficulty concentrating, low mood, dry skin and hair and a croaky voice. Symptoms usually develop slowly and get worse over time.

Because the pituitary controls other hormones as well, there may be signs of those deficiencies. The Foundation describes, for example, tiredness with low blood pressure and frequent infections when ACTH is low, irregular or absent periods and low libido when the sex hormone signals are low, and excessive thirst and passing a lot of urine when vasopressin is low. The Foundation also lists headaches among the symptoms people should report to their GP after a serious head injury, so any new or persistent headache should be mentioned to a doctor.

The NHS explains that for a suspected underactive thyroid, a GP may check TSH and thyroxine (T4) in the blood. The British Thyroid Foundation says an underactive thyroid is typically associated with a TSH above the reference range and a free T4 below it. In central hypothyroidism, however, the pituitary is the faulty part, so TSH is not raised as it normally would be. TSH may be low or in the normal range even while thyroxine is low. A test that relies on TSH alone can therefore be falsely reassuring, which is why a low or low-normal free T4 should lead to further assessment rather than being dismissed.

If hypopituitarism is suspected, the Pituitary Foundation says blood tests measure hormone levels, often first thing in the morning when many pituitary hormones are highest, and some hormones need stimulation tests in which the body is prompted to release the hormone and the response is measured. Because a tumour is the most common cause, your endocrinologist may also arrange imaging of the pituitary, such as an MRI scan, although the Foundation’s page does not detail this. If you have had a serious head injury, the Foundation advises contacting your GP if symptoms persist 12 months afterwards.

The Pituitary Foundation states that treatment usually means replacing the missing hormones, and that for people with TSH deficiency this is levothyroxine, taken as a tablet first thing in the morning on an empty stomach. For most people, hormone replacement is life-long. It notes that ongoing blood tests are needed to make sure the dose is right and that the amount needed may vary through life.

An important practical point follows from the nature of the condition. Because TSH is made by the pituitary, it cannot be used as the dial for adjusting levothyroxine in the way it is in ordinary hypothyroidism, so your endocrinology team will decide how to monitor you, usually by looking at thyroid hormone levels and your symptoms. This is why care is led by an endocrinologist rather than managed only by tablet-dose changes in primary care.

There is another important safety point. The Pituitary Foundation notes that more than one hormone may be affected and describes cortisol replacement with hydrocortisone for ACTH deficiency, and the NHS’s advice on underactive thyroid warns that a rare severe form (myxoedema coma) is an emergency that needs hospital treatment. Seek urgent help if you are severely drowsy, confused, very cold or unwell; call 999 or attend A&E.

The Pituitary Foundation notes that it is common to feel a swing of emotions with a pituitary condition and that hormone replacement cannot exactly mimic the pituitary gland. It also advises continuing your hormone replacement even if you are unwell unless a doctor says otherwise, and not doubling doses after a missed tablet.

Care in the NHS is usually through an endocrinology clinic, and some people also use private endocrinology consultations for assessment. Support is available from the Pituitary Foundation and the British Thyroid Foundation, which offer information, helplines and peer support.