Sub-clinical is a medical term referring to a disease process that has initiated but has not yet manifested symptoms.

One example is sub-clinical hypothyroidism

Hypothyroidism

Hypothyroidism is a condition in which the thyroid gland does not make enough thyroid hormone.Iodine deficiency is the most common cause of hypothyroidism worldwide but it can be caused by other causes such as several conditions of the thyroid gland or, less commonly, the pituitary gland or...

, where a patient will manifest none of the typical symptoms of hypothyroidism, and blood tests will show a normal T3

Triiodothyronine

Triiodothyronine, C15H12I3NO4, also known as T3, is a thyroid hormone. It affects almost every physiological process in the body, including growth and development, metabolism, body temperature, and heart rate....

 and T4

Thyroxine

Thyroxine, or 3,5,3',5'-tetraiodothyronine , a form of thyroid hormones, is the major hormone secreted by the follicular cells of the thyroid gland.-Synthesis and regulation:...

, but an elevated TSH

Thyroid-stimulating hormone

Thyrotrophin-stimulating hormone is a peptide hormone synthesized and secreted by thyrotrope cells in the anterior pituitary gland, which regulates the endocrine function of the thyroid gland.- Physiology :...

--indicating that the thyroid gland is requiring a greater-than-normal stimulus to produce a normal level of thyroid hormone

Thyroid hormone

The thyroid hormones, thyroxine and triiodothyronine , are tyrosine-based hormones produced by the thyroid gland primarily responsible for regulation of metabolism. An important component in the synthesis of thyroid hormones is iodine. The major form of thyroid hormone in the blood is thyroxine ,...

.

Another frequently cited example is a sub-clinical thiamine

Thiamine

Thiamine or thiamin or vitamin B1 , named as the "thio-vitamine" is a water-soluble vitamin of the B complex. First named aneurin for the detrimental neurological effects if not present in the diet, it was eventually assigned the generic descriptor name vitamin B1. Its phosphate derivatives are...

 deficiency. A patient with a sub-clinical thimamine deficiency may have low levels of thiamine, but not be manifesting any of the symptoms. However, the administration of intravenous glucose may induce Wernicke-Korsakoff Syndrome

Wernicke-Korsakoff syndrome

Wernicke–Korsakoff syndrome is a manifestation of thiamine deficiency, or beriberi. This is usually secondary to alcohol abuse...

 or beri-beri by exacerbating the already underlying thiamine deficiency, as thiamine is consumed during normal glucose metabolism.