Hypothyroidism

What it is

Hypothyroidism is read off the thyroid profile as a pattern of TSH against T4. A high TSH with a low T4 is primary hypothyroidism. A high TSH with a normal T4 is subclinical hypothyroidism. A low TSH with a low T4 is central, or secondary, hypothyroidism, and it points to pituitary or hypothalamic pathology from non-secretory lesions, written elsewhere as a non-secretory pituitary adenoma. When TSH and T4 are both low or both normal in a patient ill for another reason, the label is sick euthyroid syndrome, seen in non-thyroidal illness, and the TFT is repeated in 6-8 weeks.

Causes and risk

Primary hypothyroidism is set against autoimmune thyroiditis on the thyroid function grid. When a goiter is assessed, a high TSH is followed by free T4, and TPO antibodies are measured next: a positive TPO means Hashimoto's thyroiditis, while a negative TPO with a high TSH is called seronegative Hashimoto's thyroiditis, or else points to biosynthetic defects in thyroid hormone synthesis or iodine utilization, or to an infiltrative disorder of the thyroid. Secondary hypothyroidism follows a non-secretory pituitary adenoma. Myxedema coma is precipitated by cold exposure, infection, trauma or sedatives.

The thyroiditis table sets Hashimoto's thyroiditis apart from the other syndromes. Its cause is autoimmune; it runs at all ages with a peak at 30-50, and the sex ratio F:M is 8-9:1. The pathological findings are lymphocytic infiltration, germinal centers and fibrosis, the thyroid function is hypothyroidism, and TPO antibodies are high titer and persistent. The ESR, erythrocyte sedimentation rate, is normal and the 24-hour uptake is variable, and that last pair is what separates it from the painful subacute form, where the ESR is high and the uptake is under 5%.